Show model/reasoning hint when switching modes (#12307)

## Summary
- show an info message when switching collaboration modes changes the
effective model or reasoning
- include the target mode in the message (for example `... for Plan
mode.`)
- add TUI tests for model-change and reasoning-only change notifications
on mode switch

<img width="715" height="184" alt="Screenshot 2026-02-20 at 2 01 40 PM"
src="https://github.com/user-attachments/assets/18d1beb3-ab87-4e1c-9ada-a10218520420"
/>

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

@@ -3584,6 +3584,66 @@ async fn collab_mode_shift_tab_cycles_only_when_enabled_and_idle() {
     assert_eq!(chat.active_collaboration_mode_kind(), before);
 }
 
+#[tokio::test]
+async fn mode_switch_surfaces_model_change_notification_when_effective_model_changes() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(Some("gpt-5")).await;
+    chat.set_feature_enabled(Feature::CollaborationModes, true);
+    let default_model = chat.current_model().to_string();
+
+    let mut plan_mask =
+        collaboration_modes::mask_for_kind(chat.models_manager.as_ref(), ModeKind::Plan)
+            .expect("expected plan collaboration mode");
+    plan_mask.model = Some("gpt-5.1-codex-mini".to_string());
+    chat.set_collaboration_mask(plan_mask);
+
+    let plan_messages = drain_insert_history(&mut rx)
+        .iter()
+        .map(|lines| lines_to_single_string(lines))
+        .collect::<Vec<_>>()
+        .join("\n");
+    assert!(
+        plan_messages.contains("Model changed to gpt-5.1-codex-mini medium for Plan mode."),
+        "expected Plan-mode model switch notice, got: {plan_messages:?}"
+    );
+
+    let default_mask = collaboration_modes::default_mask(chat.models_manager.as_ref())
+        .expect("expected default collaboration mode");
+    chat.set_collaboration_mask(default_mask);
+
+    let default_messages = drain_insert_history(&mut rx)
+        .iter()
+        .map(|lines| lines_to_single_string(lines))
+        .collect::<Vec<_>>()
+        .join("\n");
+    let expected_default_message =
+        format!("Model changed to {default_model} default for Default mode.");
+    assert!(
+        default_messages.contains(&expected_default_message),
+        "expected Default-mode model switch notice, got: {default_messages:?}"
+    );
+}
+
+#[tokio::test]
+async fn mode_switch_surfaces_reasoning_change_notification_when_model_stays_same() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(Some("gpt-5.3-codex")).await;
+    chat.set_feature_enabled(Feature::CollaborationModes, true);
+    chat.set_reasoning_effort(Some(ReasoningEffortConfig::High));
+
+    let plan_mask = collaboration_modes::plan_mask(chat.models_manager.as_ref())
+        .expect("expected plan collaboration mode");
+    chat.set_collaboration_mask(plan_mask);
+
+    let plan_messages = drain_insert_history(&mut rx)
+        .iter()
+        .map(|lines| lines_to_single_string(lines))
+        .collect::<Vec<_>>()
+        .join("\n");
+    assert!(
+        plan_messages.contains("Model changed to gpt-5.3-codex medium for Plan mode."),
+        "expected reasoning-change notice in Plan mode, got: {plan_messages:?}"
+    );
+}
+
 #[tokio::test]
 async fn collab_slash_command_opens_picker_and_updates_mode() {
     let (mut chat, mut rx, mut op_rx) = make_chatwidget_manual(None).await;
@@ -3649,7 +3709,12 @@ async fn plan_slash_command_switches_to_plan_mode() {
 
     chat.dispatch_command(SlashCommand::Plan);
 
-    assert!(rx.try_recv().is_err(), "plan should not emit an app event");
+    while let Ok(event) = rx.try_recv() {
+        assert!(
+            matches!(event, AppEvent::InsertHistoryCell(_)),
+            "plan should not emit a non-history app event: {event:?}"
+        );
+    }
     assert_eq!(chat.active_collaboration_mode_kind(), ModeKind::Plan);
     assert_eq!(chat.current_collaboration_mode(), &initial);
 }