feat: add exit slash command alias for quit (#6002)

## Summary
- add the `/exit` slash command alongside `/quit` and reuse shared exit
handling
- refactor the chat widget to funnel quit, exit, logout, and shutdown
flows through a common `request_exit` helper
- add focused unit tests that confirm both `/quit` and `/exit` send an
`ExitRequest`

## Testing
- `just fmt`
- `just fix -p codex-tui`
- `cargo test -p codex-tui`


------
https://chatgpt.com/codex/tasks/task_i_6903d5a8f47c8321bf180f031f2fa330

codex-rs/tui/src/chatwidget/tests.rs

@@ -851,6 +851,24 @@ fn slash_init_skips_when_project_doc_exists() {
     );
 }
 
+#[test]
+fn slash_quit_requests_exit() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual();
+
+    chat.dispatch_command(SlashCommand::Quit);
+
+    assert_matches!(rx.try_recv(), Ok(AppEvent::ExitRequest));
+}
+
+#[test]
+fn slash_exit_requests_exit() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual();
+
+    chat.dispatch_command(SlashCommand::Exit);
+
+    assert_matches!(rx.try_recv(), Ok(AppEvent::ExitRequest));
+}
+
 #[test]
 fn slash_undo_sends_op() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual();