feat(flags): add $feature_flag_has_experiment to $feature_flag_called events

[haacked]

💡 Motivation and Context

Part of a cross-SDK effort to shrink $feature_flag_called events. This first phase adds a $feature_flag_has_experiment boolean property to every $feature_flag_called event, reflecting the server's has_experiment signal: metadata.has_experiment in the /flags?v=2 response and has_experiment on /api/feature_flag/local_evaluation flag definitions. When the server does not report the field (older deployments, bootstrapped flags), the property is omitted, so it is tri-state: true, false, or absent (unknown).

This lets ingestion distinguish experiment-linked flag events (which need the full property set for exposure analysis) from the rest, and lets us measure the split before a later phase strips the expensive properties ($feature/<key>, $feature_flag_payload, per-event system metadata) from non-experiment flag events. This PR intentionally minimizes nothing: no properties are removed, no config options are added, and dedupe behavior is unchanged.

Changes

• FeatureFlagMetadata.HasExperiment (has_experiment) on the /flags DTO and the same property on LocalFeatureFlag for /local_evaluation; both factories thread it onto the shared FeatureFlag record (included in record equality).
• Both send paths funnel through BuildFeatureFlagCalledProperties, which now always sets $feature_flag_has_experiment; synthetic missing/error flags report false. Legacy v3 responses and missing metadata safely default to false.
• PublicAPI.Unshipped.txt updated (RS0016); minor changeset.

💚 How did you test it?

dotnet build PostHog.sln: 0 errors, 0 warnings. dotnet test: 1,012 passed (UnitTests 944 + AspNetCore 49 + PostHog.AI 19), 0 failures. Two new theories cover true / false / absent from flags metadata and local evaluation, and 20 exact-JSON expectations were updated. bin/fmt clean.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

[github-actions]

posthog-dotnet Compliance Report

Date: 2026-07-15 23:25:47 UTC
Duration: 4486ms

✅ All Tests Passed!

17/17 tests passed

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Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	250ms
Request Payload.Flags Request Uses V2 Query Param	✅	125ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	113ms
Request Payload.Flags Request Omits Authorization Header	✅	110ms
Request Payload.Token In Flags Body Matches Init	✅	109ms
Request Payload.Groups Round Trip	✅	113ms
Request Payload.Groups Default To Empty Object	✅	112ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	111ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	111ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	109ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	115ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	215ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	115ms
Retry Behavior.Retries Flags On 502	✅	1116ms
Retry Behavior.Retries Flags On 504	✅	1113ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	213ms

[greptile-apps]

_{Reviews (1): Last reviewed commit: "Add $feature_flag_has_experiment to $fea..." | Re-trigger Greptile}

[turnipdabeets]

LGTM! Both send paths covered (flags metadata + local evaluation), null-safe defaults to false on older deployments, and the test matrix covers true/false/absent for both sources.