tui_app_server: cancel active login before Ctrl+C exit (#15673)

## Summary

Fixes slow `Ctrl+C` exit from the ChatGPT browser-login screen in
`tui_app_server`.

## Root cause

Onboarding-level `Ctrl+C` quit bypassed the auth widget's cancel path.
That let the active ChatGPT login keep running, and in-process
app-server shutdown then waited on the stale login attempt before
finishing.

## Changes

- Extract a shared `cancel_active_attempt()` path in the auth widget
- Use that path from onboarding-level `Ctrl+C` before exiting the TUI
- Add focused tests for canceling browser-login and device-code attempts
- Add app-server shutdown cleanup that explicitly drops any active login
before draining background work

codex-rs/app-server/src/codex_message_processor.rs

@@ -1931,6 +1931,13 @@ impl CodexMessageProcessor {
         }
     }
 
+    pub(crate) async fn cancel_active_login(&self) {
+        let mut guard = self.active_login.lock().await;
+        if let Some(active_login) = guard.take() {
+            drop(active_login);
+        }
+    }
+
     pub(crate) async fn clear_all_thread_listeners(&self) {
         self.thread_state_manager.clear_all_listeners().await;
     }