feat: SDK Compliance

[jose-sequeira]

💡 Motivation and Context

We need a language-agnostic way to verify that all PostHog SDKs implement the capture API correctly. The SDK Test Harness provides a shared contract and test runner. This PR integrates it into this SDK.

This adds:

• An SDK adapter that wraps this SDK and exposes the standard REST interface for the test harness
• A CI workflow that runs compliance tests on every PR and push to the default branch
• A Dockerfile for running the adapter in the test harness environment

The test harness runs a mock PostHog server, exercises the SDK through the adapter, and verifies behavior matches the contract defined in CONTRACT.yaml (event format, retries, compression, deduplication, etc.).

#skip-changelog

💚 How did you test it?

The compliance tests run as part of this PR's CI. The adapter was also tested locally via docker-compose.

📝 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.

[github-actions]

posthog-ios Compliance Report

Date: 2026-01-28 13:11:43 UTC
Duration: 153231ms

⚠️ Some Tests Failed

17/29 tests passed, 12 failed

---

Capture Tests

⚠️ 17/29 tests passed, 12 failed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	2174ms
Format Validation.Event Has Uuid	✅	2135ms
Format Validation.Event Has Lib Properties	✅	2204ms
Format Validation.Distinct Id Is String	✅	2226ms
Format Validation.Token Is Present	✅	2189ms
Format Validation.Custom Properties Preserved	✅	2324ms
Format Validation.Event Has Timestamp	✅	2152ms
Retry Behavior.Retries On 503	❌	7274ms
Retry Behavior.Does Not Retry On 400	✅	4299ms
Retry Behavior.Does Not Retry On 401	✅	4232ms
Retry Behavior.Respects Retry After Header	❌	7240ms
Retry Behavior.Implements Backoff	❌	17242ms
Retry Behavior.Retries On 500	❌	7204ms
Retry Behavior.Retries On 502	❌	7201ms
Retry Behavior.Retries On 504	❌	7208ms
Retry Behavior.Max Retries Respected	❌	17276ms
Deduplication.Generates Unique Uuids	✅	2331ms
Deduplication.Preserves Uuid On Retry	❌	7223ms
Deduplication.Preserves Uuid And Timestamp On Retry	❌	12228ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	❌	7372ms
Deduplication.No Duplicate Events In Batch	✅	2415ms
Deduplication.Different Events Have Different Uuids	✅	2284ms
Compression.Sends Gzip When Enabled	✅	2303ms
Batch Format.Uses Proper Batch Structure	✅	2184ms
Batch Format.Flush With No Events Sends Nothing	✅	2166ms
Batch Format.Multiple Events Batched Together	❌	2349ms
Error Handling.Does Not Retry On 403	✅	4132ms
Error Handling.Does Not Retry On 413	✅	4229ms
Error Handling.Retries On 408	❌	7206ms

Failures

retry_behavior.retries_on_503

Expected at least 3 requests, got 1

retry_behavior.respects_retry_after_header

Expected at least 2 requests, got 1

retry_behavior.implements_backoff

Expected at least 3 requests, got 1

retry_behavior.retries_on_500

Expected at least 2 requests, got 1

retry_behavior.retries_on_502

Expected at least 2 requests, got 1

retry_behavior.retries_on_504

Expected at least 2 requests, got 1

retry_behavior.max_retries_respected

Expected 4 requests, got 1

deduplication.preserves_uuid_on_retry

Need at least 2 requests to check retry

deduplication.preserves_uuid_and_timestamp_on_retry

Expected at least 3 requests, got 1

deduplication.preserves_uuid_and_timestamp_on_batch_retry

UUIDs changed on retry: ['019C04BA-A776-7065-B102-C9E08B43C20B'] != ['019C04BA-A7D9-7E42-8B07-5E1972DF5189', '019C04BA-A807-7594-89B6-6DE51431B533']

batch_format.multiple_events_batched_together

Expected 1 requests, got 3

error_handling.retries_on_408

Expected at least 2 requests, got 1