Add running-task exit choices to local daemon sessions (#38447)

## What changed

When `Ctrl-C` is pressed with an empty composer while a task is running in a local daemon session, show a menu that lets the user:

- cancel the task and stay in Codex;
- exit Codex while leaving the task running; or
- stop the task and exit Codex.

Do not offer background execution for side-thread tasks or when follow-up messages are queued. Preserve existing `Ctrl-C` behavior for non-daemon sessions, drafts, paste bursts, MCP startup, and returning from an active side thread.

## Testing

Add TUI tests for each exit action, menu dismissal, side threads, queued follow-ups, composer input, MCP startup, and non-daemon sessions.

GitOrigin-RevId: 539c33d3a4ff288a015bddbe9465d2bbdbe018d0

codex-rs/tui/src/app_event.rs

@@ -342,6 +342,12 @@ pub(crate) enum AppEvent {
     /// background tasks, rollout flush, or child process cleanup).
     Exit(ExitMode),
 
+    /// Apply a choice from the running-task exit menu to its originating thread.
+    RunningTaskExit {
+        action: RunningTaskExitAction,
+        thread_id: ThreadId,
+    },
+
     /// Request app-server account logout, then exit after it succeeds.
     Logout,
 
@@ -1213,6 +1219,14 @@ pub(crate) enum ExitMode {
     Immediate,
 }
 
+/// Choice made when leaving a daemon-backed task that is still running.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum RunningTaskExitAction {
+    CancelTask,
+    RunInBackground,
+    Exit,
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(crate) enum FeedbackCategory {
     BadResult,