feat(flags): switch local evaluation endpoint to /flags/definitions

[patricio-posthog]

Motivation and Context

The Rust feature flags definitions fleet now serves 100% of /api/feature_flag/local_evaluation traffic in all environments. This switches the SDK's default polling URL from the legacy Django path to the Rust endpoint's native path (/flags/definitions).

The old /api/feature_flag/local_evaluation path remains registered as a route alias on the Rust service, so older SDK versions continue to work.

How did you test it?

• Updated all fake HTTP handler URLs across 4 test files
• The /flags/definitions endpoint has been serving production traffic since 2026-04-09

[github-actions]

posthog-dotnet Compliance Report

Date: 2026-04-21 14:26:46 UTC
Duration: 260ms

⚠️ Some Tests Failed

0/1 tests passed, 1 failed

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Feature_Flags Tests

⚠️ 0/1 tests passed, 1 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	❌	43ms

Failures

request_payload.request_with_person_properties_device_id

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

[greptile-apps]

Prompt To Fix All With AI

This is a comment left during a code review.
Path: tests/TestLibrary/Fakes/FakeHttpMessageHandlerExtensions.cs
Line: 71

Comment:
**URL constant duplicated across test files**

`LocalEvaluationUrl` has no access modifier (private by default), so every test file that needs the raw URI for a custom response — `LocalFeatureFlagsLoaderTests.cs` (defines its own `static readonly Uri LocalEvaluationUrl`) and the inline `new Uri(...)` literals in `FeatureFlagsTests.cs` and `PostHogClientTests.cs` — must repeat the string. This PR updated 7 spots for a single URL change, which is the OnceAndOnlyOnce violation the simplicity rules flag.

Making this field `internal` (and verifying the UnitTests project already has `InternalsVisibleTo` access, or promoting it to `public`) would let every test reference a single source of truth and reduce future churn to one edit.

How can I resolve this? If you propose a fix, please make it concise.

_{Reviews (1): Last reviewed commit: "feat(flags): switch local evaluation end..." | Re-trigger Greptile}