Mark message processing as failed

Marks a message processing as failed by a participant. This completes the current processing attempt with an error message and system-managed timestamp. This endpoint automatically: - Sets the current attempt's completed_at timestamp (system-managed) - Sets the current attempt status to "failed" - Records the error message in the current attempt - Updates the participant's delivery status to "failed" Note: This endpoint requires an active processing attempt. If no processing attempt exists, it will return a 422 error.