chore: add targeted utils mutation tests

[marandaneto]

💡 Motivation and Context

Add targeted mutation testing coverage for posthog/utils.py so regressions in utility helpers and flag cache behavior are caught by tests.

This adds a focused mutmut configuration and CI job that only runs when the targeted mutation inputs change, with mutants/ cached across CI runs. The targeted utils tests now cover all statements in posthog/utils.py.

💚 How did you test it?

• uv run --extra test pytest posthog/test/test_utils.py posthog/test/test_size_limited_dict.py --timeout=30 -q
• uv run --extra test coverage run -m pytest posthog/test/test_utils.py posthog/test/test_size_limited_dict.py --timeout=30 -q && uv run --extra test coverage report -m posthog/utils.py
• rm -rf mutants && uv run --extra test --with mutmut mutmut run --max-children 1 && uv run --extra test --with mutmut mutmut results
• uv run --extra test pytest --timeout=30 -q
• Parsed .github/workflows/ci.yml as YAML

📝 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

[github-actions]

posthog-python Compliance Report

Date: 2026-05-16 00:03:28 UTC
Duration: 4674437ms

⚠️ Some Tests Failed

29/45 tests passed, 16 failed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	518ms
Format Validation.Event Has Uuid	✅	1507ms
Format Validation.Event Has Lib Properties	✅	1507ms
Format Validation.Distinct Id Is String	✅	1507ms
Format Validation.Token Is Present	✅	1508ms
Format Validation.Custom Properties Preserved	✅	1507ms
Format Validation.Event Has Timestamp	✅	1507ms
Retry Behavior.Retries On 503	✅	9519ms
Retry Behavior.Does Not Retry On 400	✅	3506ms
Retry Behavior.Does Not Retry On 401	✅	3507ms
Retry Behavior.Respects Retry After Header	✅	9515ms
Retry Behavior.Implements Backoff	✅	23518ms
Retry Behavior.Retries On 500	✅	7517ms
Retry Behavior.Retries On 502	✅	7512ms
Retry Behavior.Retries On 504	✅	7513ms
Retry Behavior.Max Retries Respected	✅	23528ms
Deduplication.Generates Unique Uuids	✅	1499ms
Deduplication.Preserves Uuid On Retry	✅	7515ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	14519ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7512ms
Deduplication.No Duplicate Events In Batch	✅	1504ms
Deduplication.Different Events Have Different Uuids	✅	1508ms
Compression.Sends Gzip When Enabled	✅	1508ms
Batch Format.Uses Proper Batch Structure	✅	1507ms
Batch Format.Flush With No Events Sends Nothing	✅	1006ms
Batch Format.Multiple Events Batched Together	✅	1505ms
Error Handling.Does Not Retry On 403	✅	3510ms
Error Handling.Does Not Retry On 413	✅	3509ms
Error Handling.Retries On 408	✅	7514ms

Feature_Flags Tests

⚠️ 0/16 tests passed, 16 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	❌	506ms
Request Payload.Flags Request Uses V2 Query Param	❌	300398ms
Request Payload.Flags Request Hits Flags Path Not Decide	❌	301003ms
Request Payload.Flags Request Omits Authorization Header	❌	300956ms
Request Payload.Token In Flags Body Matches Init	❌	300992ms
Request Payload.Groups Round Trip	❌	301007ms
Request Payload.Groups Default To Empty Object	❌	301044ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	❌	300998ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	❌	300918ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	301081ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	❌	301004ms
Request Lifecycle.No Flags Request On Init Alone	❌	301000ms
Request Lifecycle.No Flags Request On Normal Capture	❌	300994ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	❌	300946ms
Request Lifecycle.Mock Response Value Is Returned To Caller	❌	301059ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	❌	300997ms

Failures

request_payload.request_with_person_properties_device_id

Field 'token' not found in /flags request body at path 'token'. Available keys: ['distinct_id', 'groups', 'person_properties', 'group_properties', 'geoip_disable', 'device_id', 'flag_keys_to_evaluate', 'sentAt', 'api_key']

request_payload.flags_request_uses_v2_query_param

No error message

request_payload.flags_request_hits_flags_path_not_decide

No error message

request_payload.flags_request_omits_authorization_header

No error message

request_payload.token_in_flags_body_matches_init

No error message

request_payload.groups_round_trip

No error message

request_payload.groups_default_to_empty_object

No error message

request_payload.person_properties_distinct_id_auto_populated_when_caller_omits_it

No error message

request_payload.disable_geoip_false_propagates_as_geoip_disable_false

No error message

request_payload.disable_geoip_omitted_defaults_to_false

No error message

request_payload.flag_keys_to_evaluate_contains_only_requested_key

No error message

request_lifecycle.no_flags_request_on_init_alone

No error message

request_lifecycle.no_flags_request_on_normal_capture

No error message

request_lifecycle.two_flag_calls_produce_two_remote_requests

No error message

request_lifecycle.mock_response_value_is_returned_to_caller

No error message

side_effect_events.get_feature_flag_captures_feature_flag_called_event

No error message