feat(flags): Add more details such as version, id, and reason to $feature_flag_called events

[haacked]

Add support for /decide?v=4 (see PostHog/posthog#29751). Similar to how it's done in PostHog/posthog-js-lite#427.

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

[greptile-apps]

PR Summary

Here's my review of the changes to add /decide?v=4 support and enhance feature flag events:

This PR adds support for the new /decide?v=4 endpoint and enriches feature flag events with additional metadata like version, ID, and evaluation reason.

Key points:

• New types.py introduces structured data classes for flag metadata but has potential type safety issues with None returns in from_json methods
• The get_value() assertion in FeatureFlag could fail in production and should be replaced with safer validation
• normalize_decide_response() modifies input dictionaries which could cause side effects
• Test coverage for the v4 decide endpoint functionality in test_request.py needs expansion
• The to_payloads() return type annotation incorrectly specifies str when it should be Any

The changes maintain backwards compatibility while adding important new capabilities, but would benefit from addressing these type safety and testing gaps.

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[dmarticus]

this is just good clean fun

[haacked]

I’ll merge after bumping minor version and adding changelog notes