Deleting a thread mid-answer no longer reports an error

Deleting a thread while an agent was still answering in it produced an error report, even though nothing had actually gone wrong — the answer simply had nowhere left to go. The round is now recognised as one that ended because its thread was being removed, and it settles quietly.

Nothing about a genuine failure changes. A round that really does fail is still surfaced exactly as before, so this removes noise without removing a signal.