Closes#32951
## Summary
When an agent notification was shown and the `AcpThreadView` was dropped
(e.g., by closing the project window or navigating to a new thread), the
notification would become orphaned and undismissable because the
subscriptions handling dismiss events were dropped along with the thread
view.
## Fix
Added an `on_release` callback that closes all notification windows when
the thread view is dropped. This ensures notifications are always
cleaned up properly.
## Testing
Added `test_notification_closed_when_thread_view_dropped` to verify
notifications are closed when the thread view is dropped.
Release Notes:
- Fixed agent notification getting stuck and becoming undismissable when
the project window is closed or when navigating to a new thread
be1f824a35