The KeyboardInterrupt handler in run_gateway() was the only exit path
that still used bare 'return' instead of _hard_exit_after_gateway_teardown().
While less common than service-managed restarts, a console Ctrl+C still
leaves the process vulnerable to the same Python finalization hang on
non-daemon worker threads (cron ThreadPoolExecutor jobs). Route it through
the same backstop, with a 'return' guard for test stubs that don't raise
on code 0 (production os._exit never returns).
