Fix /review interrupt and TUI exit wedges (#18921)

Addresses #11267

## Summary
`/review` can be interrupted while it is still spawning the review
sub-agent. That spawn path lives in `codex-core` and did not observe the
task cancellation token until after `Codex::spawn` returned, so an
interrupted review could keep building a child session and leave the TUI
in a wedged state.

The TUI exit path also waited indefinitely for app-server
`thread/unsubscribe`, which made Ctrl+C look broken if the app-server
was already stuck. This makes interactive delegate startup
cancellation-aware and bounds the TUI shutdown-first unsubscribe wait
with a short UI escape-hatch timeout.

## Testing
I reproed the hang using the steps in the bug report. Confirmed hang no
longer exists after fix.

codex-rs/core/src/codex_delegate_tests.rs

@@ -153,6 +153,32 @@ async fn forward_ops_preserves_submission_trace_context() {
         .expect("forward_ops join error");
 }
 
+#[tokio::test]
+async fn run_codex_thread_interactive_respects_pre_cancelled_spawn() {
+    let (parent_session, parent_ctx, _rx_events) =
+        crate::session::tests::make_session_and_context_with_rx().await;
+    let cancel_token = CancellationToken::new();
+    cancel_token.cancel();
+
+    let result = timeout(
+        Duration::from_secs(/*secs*/ 1),
+        run_codex_thread_interactive(
+            parent_ctx.config.as_ref().clone(),
+            Arc::clone(&parent_session.services.auth_manager),
+            Arc::clone(&parent_session.services.models_manager),
+            parent_session,
+            parent_ctx,
+            cancel_token,
+            SubAgentSource::Review,
+            /*initial_history*/ None,
+        ),
+    )
+    .await
+    .expect("cancelled delegate spawn should not hang");
+
+    assert!(matches!(result, Err(CodexErr::TurnAborted)));
+}
+
 #[tokio::test]
 async fn handle_request_permissions_uses_tool_call_id_for_round_trip() {
     let (parent_session, parent_ctx, rx_events) =