chore: update endpoint for flags local eval

[patricio-posthog]

💡 Motivation and Context

The Rust feature flags definitions fleet now serves 100% of /api/feature_flag/local_evaluation traffic in all environments (dev, prod-us, prod-eu) via Contour weighted routing. We saw zero errors after ramping 10% → 50% → 100% over the past week.

This PR switches the SDK's default polling URL from the legacy Django path (/api/feature_flag/local_evaluation/) to the Rust endpoint's native path (/flags/definitions). This also removes the intermediate POSTHOG_LOCAL_EVALUATION_ENDPOINT env var override and fallback logic that were added as temporary migration scaffolding.

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?

• Existing tests pass (the removed TestLocalEvalEndpointConfig class tested the env var/fallback machinery that no longer exists)
• The /flags/definitions endpoint has been serving production traffic since 2026-04-09 (PostHog internal SDK) and 100% of external traffic since 2026-04-15

📝 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 sampo add to generate a changeset file
• Added the release label to the PR

[greptile-apps]

Prompt To Fix All With AI

This is a comment left during a code review.
Path: posthog/test/test_feature_flags.py
Line: 3782

Comment:
**No test for new `/flags/definitions` URL**

`TestLocalEvalEndpointConfig` was the only test asserting what URL `_fetch_feature_flags_from_api` passes to `get()`. Now that it is removed there is nothing to prevent a silent regression if the hardcoded path is accidentally changed. A minimal parameterised replacement (even a single case) that checks `patch_get.call_args[0][1].startswith("/flags/definitions?")` would close this gap while keeping the test suite honest about the URL the SDK actually calls.

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

_{Reviews (1): Last reviewed commit: "chore: update endpoint for flags local e..." | Re-trigger Greptile}

[github-actions]

posthog-python Compliance Report

Date: 2026-04-15 15:16:34 UTC
Duration: 159977ms

⚠️ Some Tests Failed

29/30 tests passed, 1 failed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	516ms
Format Validation.Event Has Uuid	✅	1506ms
Format Validation.Event Has Lib Properties	✅	1506ms
Format Validation.Distinct Id Is String	✅	1507ms
Format Validation.Token Is Present	✅	1505ms
Format Validation.Custom Properties Preserved	✅	1505ms
Format Validation.Event Has Timestamp	✅	1505ms
Retry Behavior.Retries On 503	✅	9517ms
Retry Behavior.Does Not Retry On 400	✅	3506ms
Retry Behavior.Does Not Retry On 401	✅	3508ms
Retry Behavior.Respects Retry After Header	✅	9513ms
Retry Behavior.Implements Backoff	✅	23519ms
Retry Behavior.Retries On 500	✅	7509ms
Retry Behavior.Retries On 502	✅	7509ms
Retry Behavior.Retries On 504	✅	7505ms
Retry Behavior.Max Retries Respected	✅	23521ms
Deduplication.Generates Unique Uuids	✅	1508ms
Deduplication.Preserves Uuid On Retry	✅	7510ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	14525ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7507ms
Deduplication.No Duplicate Events In Batch	✅	1502ms
Deduplication.Different Events Have Different Uuids	✅	1508ms
Compression.Sends Gzip When Enabled	✅	1506ms
Batch Format.Uses Proper Batch Structure	✅	1507ms
Batch Format.Flush With No Events Sends Nothing	✅	1004ms
Batch Format.Multiple Events Batched Together	✅	1504ms
Error Handling.Does Not Retry On 403	✅	3509ms
Error Handling.Does Not Retry On 413	✅	3507ms
Error Handling.Retries On 408	✅	7509ms

Feature_Flags Tests

⚠️ 0/1 tests passed, 1 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	❌	506ms

Failures

request_payload.request_with_person_properties_device_id

404, message='NOT FOUND', url='http://sdk-adapter:8080/get_feature_flag'