test(tui): restore missing tokio test attribute

Mark `set_model_updates_active_collaboration_mask` as a Tokio test again so it is referenced during test builds. This fixes the dead-code clippy failure that broke the manual verify workflow on main.

Author: ignatremizov

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

@@ -6075,6 +6075,7 @@ async fn collaboration_modes_defaults_to_code_on_startup() {
     assert_eq!(chat.current_model(), resolved_model);
 }
 
+#[tokio::test]
 async fn set_model_updates_active_collaboration_mask() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(Some("gpt-5.1")).await;
     chat.set_feature_enabled(Feature::CollaborationModes, true);