Make less decide requests when enriching events

[haacked]

When sendFeatureFlags is true, the client makes a Decide requests to get the latest feature flags in order to enrich the captured event with the current feature flag values.

However, when processing a batch, it doesn't make sense to make a decide request multiple times for the same distinctId. This PR adds an in-memory cache that lasts for the duration of the batch.

So far, I don't think a lot of the other client libraries actually use the /batch endpoint to batch requests. And if they do, I don't think they're caching /decide requests. This is a change I'd like to consider doing for all of the clients.

[Copilot]

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/PostHog/Library/AsyncBatchHandler.cs:92

• [nitpick] Consider using a more descriptive parameter name instead of '_' in the lambda to improve code clarity, for example 'context'.

public bool Enqueue(Task<TItem> item) => Enqueue(new BatchItem<TItem, TBatchContext>(_ => item));

src/PostHog/PostHogClient.cs:157

• [nitpick] Consider extracting the inline lambda 'BatchTask' into a well-named private method to improve readability, particularly for the ternary logic.

Task<CapturedEvent> BatchTask(CapturedEventBatchContext context) => sendFeatureFlags ? AddFreshFeatureFlagDataAsync(context.FeatureFlagCache, distinctId, groups, capturedEvent) : _featureFlagsLoader.IsLoaded && eventName != "$feature_flag_called" ? AddLocalFeatureFlagDataAsync(distinctId, groups, capturedEvent) : Task.FromResult(capturedEvent);

[greptile-apps]

PR Summary

This PR optimizes feature flag enrichment by adding in-memory caching to avoid duplicate decide calls during batch processing.

• Updated src/PostHog/PostHogClient.cs to integrate a batch context using a FallbackFeatureFlagCache.
• Added src/PostHog/Capture/CapturedEventBatchContext.cs to encapsulate cache context for batch processing.
• Introduced src/PostHog/Features/MemoryFeatureFlagCache.cs and FallbackFeatureFlagCache.cs for caching.
• Refactored src/PostHog/Library/AsyncBatchHandler.cs to accept a batch context, reducing redundant decide requests.
• Minor syntax improvement in src/PostHog/Config/PostHogOptions.cs using target-typed new.

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