chore: Add FDv2 Polling Data Source (#190)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Implements FDv2 polling (requestor + data source) with event-array
parsing, ETag/304 handling, status updates, env-id header extraction,
and comprehensive tests.
> 
> - **FDv2 Polling**:
> - **Data Source**: New
`Internal/FDv2DataSources/FDv2PollingDataSource.cs` implementing
periodic polling, processing protocol actions, applying transactional
changesets, marking initialization, updating `DataSourceStatus`, and
handling JSON/HTTP errors.
> - **Requestor**: New
`Internal/FDv2DataSources/FDv2PollingRequestor.cs` building requests
with selector query params, ETag caching, 304 handling, header capture
(including `x-ld-envid`), and deserializing polling `events`.
> - **Interface/Models**: New `IFDv2PollingRequestor.cs` defining
`FDv2PollingResponse` and requestor interface.
> - **Event Parsing**: `Internal/FDv2Payloads/FDv2Event.cs` adds
`DeserializeEventsArray` and poll-event converter usage for parsing
`events` arrays with strict validation.
> - **Streaming**:
> - Minor imports added in `FDv2StreamingDataSource.cs` (no functional
change in this diff).
> - **Misc**:
>   - Minor cleanup in `Internal/DataSources/FeatureRequestor.cs`.
> - **Tests**:
> - Extensive tests in
`test/Internal/FDv2DataSources/FDv2PollingDataSourceTest.cs` covering
init, full/partial updates, deletes, 304 behavior, error/status
transitions, selector propagation, and env-id extraction.
> - Additional tests in `test/Internal/FDv2Payloads/FDv2PayloadsTest.cs`
for `DeserializeEventsArray` and valid/invalid payload scenarios.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
69b18d1. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: kinyoklion

pkgs/sdk/server/src/Internal/DataSources/FeatureRequestor.cs

@@ -98,7 +98,7 @@ private async Task<BytesWithHeaders> GetAsync(Uri path)
                             _log.Debug("Get all flags returned 304: not modified");
                             return null;
                         }
-                        //We ensure the status code after checking for 304, because 304 isn't considered success
+
                         if (!response.IsSuccessStatusCode)
                         {
                             throw new UnsuccessfulResponseException((int)response.StatusCode);

pkgs/sdk/server/src/Internal/FDv2DataSources/FDv2PollingDataSource.cs

@@ -0,0 +1,238 @@
+using System;
+using System.Linq;
+using System.Text.Json;
+using System.Threading;
+using System.Threading.Tasks;
+using LaunchDarkly.Logging;
+using LaunchDarkly.Sdk.Internal;
+using LaunchDarkly.Sdk.Internal.Concurrent;
+using LaunchDarkly.Sdk.Internal.Http;
+using LaunchDarkly.Sdk.Server.Interfaces;
+using LaunchDarkly.Sdk.Server.Subsystems;
+
+namespace LaunchDarkly.Sdk.Server.Internal.FDv2DataSources
+{
+    internal sealed class FDv2PollingDataSource : IDataSource
+    {
+        internal delegate Selector SelectorSource();
+
+        private readonly IFDv2PollingRequestor _requestor;
+        private readonly IDataSourceUpdates _dataSourceUpdates;
+        private readonly TaskExecutor _taskExecutor;
+        private readonly TimeSpan _pollInterval;
+        private readonly AtomicBoolean _initialized = new AtomicBoolean(false);
+        private readonly TaskCompletionSource<bool> _initTask;
+        private readonly Logger _log;
+        private readonly FDv2ProtocolHandler _protocolHandler = new FDv2ProtocolHandler();
+        private readonly object _protocolLock = new object();
+        private readonly SelectorSource _selectorSource;
+
+        private CancellationTokenSource _canceler;
+        private string _environmentId;
+
+        internal FDv2PollingDataSource(
+            LdClientContext context,
+            IDataSourceUpdates dataSourceUpdates,
+            IFDv2PollingRequestor requestor,
+            TimeSpan pollInterval,
+            SelectorSource selectorSource
+        )
+        {
+            _requestor = requestor;
+            _dataSourceUpdates = dataSourceUpdates;
+            _taskExecutor = context.TaskExecutor;
+            _pollInterval = pollInterval;
+            _selectorSource = selectorSource;
+            _initTask = new TaskCompletionSource<bool>();
+            _log = context.Logger.SubLogger(LogNames.FDv2DataSourceSubLog);
+
+            _log.Debug("Created LaunchDarkly FDv2 polling data source");
+        }
+
+        public bool Initialized => _initialized.Get();
+
+        public Task<bool> Start()
+        {
+            lock (this)
+            {
+                // If we have a canceler, then the source has already been started.
+                if (_canceler != null) return _initTask.Task;
+
+                _log.Info("Starting LaunchDarkly FDv2 polling with interval: {0} milliseconds",
+                    _pollInterval.TotalMilliseconds);
+                _canceler = _taskExecutor.StartRepeatingTask(TimeSpan.Zero,
+                    _pollInterval, UpdateTaskAsync);
+            }
+
+            return _initTask.Task;
+        }
+
+        private async Task UpdateTaskAsync()
+        {
+            _log.Debug("Polling LaunchDarkly for feature flag updates");
+            try
+            {
+                var selector = _selectorSource();
+                var response = await _requestor.PollingRequestAsync(selector);
+
+                if (response == null)
+                {
+                    // This means we got a cached response (304), so we are valid.
+                    // We could have previously been interrupted, so we need to return to valid even
+                    // if there are no changes.
+                    _dataSourceUpdates.UpdateStatus(DataSourceState.Valid, null);
+                    return;
+                }
+
+                if (response.Value.Headers != null)
+                {
+                    _environmentId = response.Value.Headers.FirstOrDefault(item =>
+                            item.Key.ToLower() == HeaderConstants.EnvironmentId).Value
+                        ?.FirstOrDefault();
+                }
+
+                ProcessPollingResponse(response.Value);
+            }
+            catch (UnsuccessfulResponseException ex)
+            {
+                var errorInfo = DataSourceStatus.ErrorInfo.FromHttpError(ex.StatusCode);
+
+                if (HttpErrors.IsRecoverable(ex.StatusCode))
+                {
+                    _log.Warn(HttpErrors.ErrorMessage(ex.StatusCode, "polling request", "will retry"));
+                    _dataSourceUpdates.UpdateStatus(DataSourceState.Interrupted, errorInfo);
+                }
+                else
+                {
+                    _log.Error(HttpErrors.ErrorMessage(ex.StatusCode, "polling request", ""));
+                    _dataSourceUpdates.UpdateStatus(DataSourceState.Off, errorInfo);
+                    try
+                    {
+                        _initTask.SetResult(true);
+                    }
+                    catch (InvalidOperationException)
+                    {
+                        // the task was already set - nothing more to do
+                    }
+
+                    ((IDisposable)this).Dispose();
+                }
+            }
+            catch (JsonException ex)
+            {
+                _log.Error("Polling request received malformed data: {0}", LogValues.ExceptionSummary(ex));
+                _dataSourceUpdates.UpdateStatus(DataSourceState.Interrupted,
+                    new DataSourceStatus.ErrorInfo
+                    {
+                        Kind = DataSourceStatus.ErrorKind.InvalidData,
+                        Time = DateTime.Now
+                    });
+            }
+            catch (Exception ex)
+            {
+                var realEx = (ex is AggregateException ae) ? ae.Flatten() : ex;
+                _log.Warn("Polling for feature flag updates failed: {0}", LogValues.ExceptionSummary(ex));
+                _log.Debug(LogValues.ExceptionTrace(ex));
+                _dataSourceUpdates.UpdateStatus(DataSourceState.Interrupted,
+                    DataSourceStatus.ErrorInfo.FromException(realEx));
+            }
+        }
+
+        private void ProcessPollingResponse(FDv2PollingResponse response)
+        {
+            lock (_protocolLock)
+            {
+                _protocolHandler.Reset();
+
+                foreach (var evt in response.Events)
+                {
+                    var action = _protocolHandler.HandleEvent(evt);
+                    ProcessProtocolAction(action);
+                }
+            }
+        }
+
+        private void HandleJsonError(string message)
+        {
+            _log.Error("LaunchDarkly polling request received invalid data: {0}", message);
+
+            var errorInfo = new DataSourceStatus.ErrorInfo
+            {
+                Kind = DataSourceStatus.ErrorKind.InvalidData,
+                Message = message,
+                Time = DateTime.Now
+            };
+            _dataSourceUpdates.UpdateStatus(DataSourceState.Interrupted, errorInfo);
+        }
+
+        private void ProcessProtocolAction(IFDv2ProtocolAction action)
+        {
+            switch (action)
+            {
+                case FDv2ActionChangeset changesetAction:
+                    ProcessChangeSet(changesetAction.Changeset);
+                    break;
+                case FDv2ActionError errorAction:
+                    _log.Error("FDv2 error event: {0} - {1}", errorAction.Id, errorAction.Reason);
+                    break;
+                case FDv2ActionGoodbye goodbyeAction:
+                    _log.Info("FDv2 server disconnecting: {0}", goodbyeAction.Reason);
+                    break;
+                case FDv2ActionInternalError internalErrorAction:
+                    _log.Error("FDv2 protocol error ({0}): {1}", internalErrorAction.ErrorType,
+                        internalErrorAction.Message);
+                    // Handle JsonError by updating status to Interrupted with InvalidData error kind
+                    if (internalErrorAction.ErrorType == FDv2ProtocolErrorType.JsonError)
+                    {
+                        HandleJsonError(internalErrorAction.Message);
+                    }
+                    break;
+                case FDv2ActionNone _:
+                    // No action needed
+                    break;
+                default:
+                    // Represents an implementation error. Actions expanded without the handling
+                    // being expanded.
+                    _log.Error("Unhandled FDv2 Protocol Action.");
+                    break;
+            }
+        }
+
+        private void ProcessChangeSet(FDv2ChangeSet fdv2ChangeSet)
+        {
+            if (!(_dataSourceUpdates is ITransactionalDataSourceUpdates transactionalDataSourceUpdates))
+                throw new InvalidOperationException("Cannot apply updates to non-transactional data source");
+
+            var dataStoreChangeSet = FDv2ChangeSetTranslator.ToChangeSet(fdv2ChangeSet, _log, _environmentId);
+
+            // If the update fails, then we wait until the next poll and try again.
+            // This is different from a streaming data source, which will need to re-start to get an initial
+            // payload.
+            if (!transactionalDataSourceUpdates.Apply(dataStoreChangeSet)) return;
+            
+            // Only mark as initialized after successfully applying a changeset
+            if (_initialized.GetAndSet(true)) return;
+            _initTask.SetResult(true);
+            _log.Info("First polling request successful");
+        }
+
+        void IDisposable.Dispose()
+        {
+            Dispose(true);
+            GC.SuppressFinalize(this);
+        }
+
+        private void Dispose(bool disposing)
+        {
+            if (!disposing) return;
+
+            Shutdown();
+        }
+
+        private void Shutdown()
+        {
+            _canceler?.Cancel();
+            _requestor.Dispose();
+        }
+    }
+}