fix(handler): add synthetic logger stacktrace

[hpouillot]

💡 Motivation and Context

Plain Logger messages do not have exception stacktraces, but Logger metadata includes source location information. This adds a synthetic single-frame stacktrace from Logger metadata so error tracking can point plain log messages back to their file, line, and function.

💚 How did you test it?

• mix test

📝 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

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with pi/coding-agent assistance. The change reuses Logger event metadata (file, line, and mfa) to create a synthetic raw stacktrace frame only when a real exception stacktrace is absent, and keeps source context enrichment compatible with the current main branch.

[github-actions]

posthog-elixir Compliance Report

Date: 2026-06-26 17:32:10 UTC
Duration: 107374ms

⚠️ Some Tests Failed

42/45 tests passed, 3 failed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	609ms
Format Validation.Event Has Uuid	✅	610ms
Format Validation.Event Has Lib Properties	✅	610ms
Format Validation.Distinct Id Is String	✅	610ms
Format Validation.Token Is Present	✅	610ms
Format Validation.Custom Properties Preserved	✅	610ms
Format Validation.Event Has Timestamp	✅	610ms
Retry Behavior.Retries On 503	✅	5615ms
Retry Behavior.Does Not Retry On 400	✅	2613ms
Retry Behavior.Does Not Retry On 401	✅	2612ms
Retry Behavior.Respects Retry After Header	✅	5616ms
Retry Behavior.Implements Backoff	✅	15626ms
Retry Behavior.Retries On 500	✅	5616ms
Retry Behavior.Retries On 502	✅	5611ms
Retry Behavior.Retries On 504	✅	5616ms
Retry Behavior.Max Retries Respected	✅	15626ms
Deduplication.Generates Unique Uuids	✅	622ms
Deduplication.Preserves Uuid On Retry	✅	5615ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10621ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5617ms
Deduplication.No Duplicate Events In Batch	✅	615ms
Deduplication.Different Events Have Different Uuids	✅	612ms
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	✅	2612ms
Error Handling.Does Not Retry On 413	✅	2612ms
Error Handling.Retries On 408	✅	5611ms

Feature_Flags Tests

⚠️ 13/16 tests passed, 3 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	9ms
Request Payload.Flags Request Uses V2 Query Param	✅	6ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	7ms
Request Payload.Flags Request Omits Authorization Header	✅	7ms
Request Payload.Token In Flags Body Matches Init	✅	7ms
Request Payload.Groups Round Trip	✅	6ms
Request Payload.Groups Default To Empty Object	✅	7ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	6ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	7ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	7ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	6ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	609ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	11ms
Request Lifecycle.Mock Response Value Is Returned To Caller	❌	7ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	❌	610ms

Failures

request_payload.disable_geoip_omitted_defaults_to_false

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

request_lifecycle.mock_response_value_is_returned_to_caller

Last action result missing field 'value'. Keys: ['error', 'success']

side_effect_events.get_feature_flag_captures_feature_flag_called_event

Expected 1 events with name '$feature_flag_called', got 0

[greptile-apps]

_{Reviews (1): Last reviewed commit: "refactor(handler): use case for syntheti..." | Re-trigger Greptile}

[hpouillot]

Updated the code your suggestion @martosaur, thanks for the review !

[hpouillot]

I switched to Logger.#{level} here, because the error message could be dynamic and cause undergrouping as we always use the exception type in fp.

[martosaur]

Oof, that's a whole different topic to be honest. Perhaps it's be best to keep this PR focused on synthetic stacktrace and not change error type logic here?

[greptile-apps]

_{Reviews (2): Last reviewed commit: "fix(handler): avoid unused reporter help..." | Re-trigger Greptile}