Forget the exit cancel of a session whose bridges stopped

[abonneth]

Made with Cursor

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Note

Low Risk
Small bookkeeping change in local session cleanup; tests cover the exit-cancel registry behavior.

Overview
When _live_sessions() drops a session because no bridges are still serving, it now also calls _deregister_exit_cancel so that session is removed from the interpreter-exit cancel registry.

That avoids exit-time (or test-simulated _cancel_sessions_at_exit) retries for sessions whose local bridges already stopped and were already handled. test_close_cancels_only_sessions_still_served was updated to seed a fake exit hook for an “ended” session and assert it is not invoked after close when that session is forgotten.

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