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/message_processor.rs

@@ -451,6 +451,10 @@ impl MessageProcessor {
         self.codex_message_processor.drain_background_tasks().await;
     }
 
+    pub(crate) async fn cancel_active_login(&self) {
+        self.codex_message_processor.cancel_active_login().await;
+    }
+
     pub(crate) async fn clear_all_thread_listeners(&self) {
         self.codex_message_processor
             .clear_all_thread_listeners()