feat(flags): implement SDK compliance adapter for posthog-elixir

[andehen]

Summary

Extend the existing sdk_compliance_adapter/ so the PostHog SDK Test Harness can exercise the feature flag and capture suites against posthog-elixir.

• Declare the capture_v0 and encoding_gzip capabilities on /health. The SDK already supports gzip via the default Req-based API client and the existing adapter passes the full capture contract — we just hadn't told the harness about it.
• Add POST /get_feature_flag, which constructs the /flags body the harness contract expects and forwards it through PostHog.FeatureFlags.flags/2.
  • distinct_id is mirrored into person_properties (the auto-added value the contract asserts on).
  • groups and group_properties default to empty maps.
  • flag_keys_to_evaluate is scoped to the requested key.
  • geoip_disable is forwarded when set.
  • api_key is auto-injected by the SDK's API client; gzip compression follows the value passed to /init.

Test plan

Locally with docker compose up --build --abort-on-container-exit against ghcr.io/posthog/sdk-test-harness:latest (digest sha256:a04e04…):

CAPTURE Tests:        29 passed | 0 failed
FEATURE_FLAGS Tests:   1 passed | 0 failed
Total: 30 | 30 passed | 0 failed

The single feature flag test currently in the suite — request_payload.request_with_person_properties_device_id — passes, including the person_properties.distinct_id auto-add assertion.

Notes

• The SDK doesn't currently surface the per-event UUID returned from bare_capture, so /capture still responds with %{success: true} only. The harness validates UUIDs via the tracked requests_made list (intercepted by SdkComplianceAdapter.TrackedClient), so all UUID-related capture tests still pass.
• force_remote is accepted but has no effect: the SDK has no local-evaluation path yet, so every /get_feature_flag already issues a fresh remote /flags request.

[github-actions]

posthog-elixir Compliance Report

Date: 2026-04-29 09:45:29 UTC
Duration: 105972ms

✅ All Tests Passed!

30/30 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	608ms
Format Validation.Event Has Uuid	✅	609ms
Format Validation.Event Has Lib Properties	✅	609ms
Format Validation.Distinct Id Is String	✅	609ms
Format Validation.Token Is Present	✅	609ms
Format Validation.Custom Properties Preserved	✅	609ms
Format Validation.Event Has Timestamp	✅	609ms
Retry Behavior.Retries On 503	✅	5614ms
Retry Behavior.Does Not Retry On 400	✅	2612ms
Retry Behavior.Does Not Retry On 401	✅	2612ms
Retry Behavior.Respects Retry After Header	✅	5616ms
Retry Behavior.Implements Backoff	✅	15612ms
Retry Behavior.Retries On 500	✅	5610ms
Retry Behavior.Retries On 502	✅	5614ms
Retry Behavior.Retries On 504	✅	5615ms
Retry Behavior.Max Retries Respected	✅	15625ms
Deduplication.Generates Unique Uuids	✅	621ms
Deduplication.Preserves Uuid On Retry	✅	5615ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10620ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5618ms
Deduplication.No Duplicate Events In Batch	✅	615ms
Deduplication.Different Events Have Different Uuids	✅	613ms
Compression.Sends Gzip When Enabled	✅	610ms
Batch Format.Uses Proper Batch Structure	✅	610ms
Batch Format.Flush With No Events Sends Nothing	✅	607ms
Batch Format.Multiple Events Batched Together	✅	615ms
Error Handling.Does Not Retry On 403	✅	2624ms
Error Handling.Does Not Retry On 413	✅	2612ms
Error Handling.Retries On 408	✅	5616ms

Feature_Flags Tests

✅ 1/1 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	8ms

[marandaneto]

i think codeql is having issues

The job was not acquired by Runner of type hosted even after multiple attempts