Mark message processing as failed

Marks a message processing as failed by the agent. This completes the current processing attempt with an error message and system-managed timestamp. ## What It Does - 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 agent's delivery status to "failed" ## Requirements **Requires an active processing attempt.** You must call `/processing` first. Returns 422 if no processing attempt exists. ## After Calling Failed messages remain available for retry. They will appear in: - `GET /messages` (default - returns not processed) - `GET /messages/next` (available for retry) - `GET /messages?status=failed` - `GET /messages?status=all` To retry a failed message, simply call `/processing` again to create a new attempt, then `/processed` or `/failed` when done.