fix flaky test falls_back_to_registered_fallback_port_when_default_po… (#20504)

…rt_is_in_use

Author: owenlin0

codex-rs/login/src/server.rs

@@ -129,7 +129,7 @@ pub struct ShutdownHandle {
 impl ShutdownHandle {
     /// Signals the login loop to terminate.
     pub fn shutdown(&self) {
-        self.shutdown_notify.notify_waiters();
+        self.shutdown_notify.notify_one();
     }
 }