chore(flags): Decide v4 support - adds version, id, reason, and requestId to $feature_flag_called events

[haacked]

This is follow on work for PostHog/posthog#29693. The work here is similar to PostHog/posthog-python#207 and PostHog/posthog-go#91.

Updates to the $feature_flag_called event

When capturing the $feature_flag_called event, additional information are now captured:

The end result is $feature_flag_called events will have additional properties:

Property Name	Descirption
$feature_flag_version	The version of the feature flag. This is visible in the diffs of the feature flag activity log.
$feature_flag_reason	The reason the feature flag matched or didn't match.
$feature_flag_id	The database id of the feature flag (Just in case a flag was deleted and then recreated with the same key, we can determine the difference.

Backwards compatibility:

The changes are all backwards compatible with /decide?v=3.

Testing

Unit tests and some manual testing

[Copilot]

Copilot reviewed 16 out of 18 changed files in this pull request and generated 3 comments.

Files not reviewed (2)

• tests/UnitTests/Fixtures/decide-api-result-v4.json: Language not supported
• tests/UnitTests/UnitTests.csproj: Language not supported

Comments suppressed due to low confidence (1)

tests/UnitTests/Json/FilterSerializationTests.cs:53

• Square bracket array literal is not valid C# syntax. Consider using curly braces and the 'new[] { ... }' syntax for array initialization.

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[greptile-apps]

PR Summary

This PR adds Decide v4 support by enhancing the payload for $feature_flag_called events with version, id, and reason fields while preserving backward compatibility with v3 responses.

• Updated /src/PostHog/Api/PostHogApiClient.cs to use the /decide?v=4 endpoint.
• Introduced the new record in /src/PostHog/Api/FeatureFlagResult.cs to encapsulate additional flag metadata.
• Modified /src/PostHog/Api/DecideApiResult.cs and /src/PostHog/Api/FeatureFlagResultExtensions.cs to normalize both v3 and v4 responses.
• Enhanced /src/PostHog/Features/FeatureFlag.cs to create a FeatureFlagWithMetadata when applicable.
• Revised tests in /tests/UnitTests/ to ensure correct handling and mapping of the new metadata.

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