Mark message as processed

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