Actually cancel the live turn on this session.

Distinct from a client disconnect: event_stream() above is a thin, disposable consumer that sse_starlette may cancel freely, while the producer (_ACTIVE_TURNS) keeps running by design so a flaky network doesn't kill a real answer. The Stop button used to only close the client's own connection — the turn kept generating/calling tools in the background regardless, which is why a "stopped" turn could still show up as stream_state='live' 15 minutes later. This is the real cancel.