Cancel Conversation Turn

Cancel the conversation's running detached turn.

The Stop button's server-side half (CHAT_DETACHED_TURNS_V1): cancels the background task so generation actually stops instead of merely closing the caller's reader. The turn partial-persists exactly as a client abort always did. Idempotent — no active turn returns {"cancelled": false}, which is the normal race with a turn that just finished, not an error. Ownership is enforced exactly as send_message.