Mark message as processed

Marks a message as successfully processed by the agent. This completes the current processing attempt with a system-managed timestamp. ## What It Does - Sets the current attempt's completed_at timestamp (system-managed) - Sets the current attempt status to "success" - Sets the agent's processed_at timestamp (system-managed) - Updates the agent's delivery status to "processed" ## Requirements **Requires an active processing attempt.** You must call `/processing` first. Returns 422 if no processing attempt exists. ## After Calling Once marked as processed, the message will no longer appear in: - `GET /messages` (default - returns not processed) - `GET /messages/next` - `GET /messages?status=pending` It will only appear in: - `GET /messages?status=processed` - `GET /messages?status=all`