Avoid rendering sub-agent activity twice in the TUI (#39049)

## Why

The app server sends sub-agent activity in both item-started and item-completed
notifications, causing the TUI to add the same activity to history twice.

## What changed

Render live sub-agent activity when the item completes instead of also rendering
it when the item starts. Preserve the existing replay behavior.

## Testing

Add a regression test that sends matching started and completed notifications
and verifies that the activity produces one history cell.

GitOrigin-RevId: 76a66f3657fe297f37067ed672ab9642997b4499

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

@@ -361,7 +361,6 @@ impl ChatWidget {
                 reasoning_effort,
                 agents_states,
             }),
-            item @ ThreadItem::SubAgentActivity { .. } => self.on_sub_agent_activity(item),
             ThreadItem::EnteredReviewMode { review, .. } if !from_replay => {
                 self.enter_review_mode_with_hint(review, /*from_replay*/ false);
             }

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__app_server_sub_agent_activity_renders_once.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/chatwidget/tests.rs
+expression: rendered
+---
+• Interacted with `/root/researcher`

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

@@ -928,6 +928,41 @@ async fn live_app_server_command_output_delta_transcript_snapshot() {
     );
 }
 
+#[tokio::test]
+async fn live_app_server_sub_agent_activity_renders_once() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    let activity = AppServerThreadItem::SubAgentActivity {
+        id: "activity-1".to_string(),
+        kind: codex_app_server_protocol::SubAgentActivityKind::Interacted,
+        agent_thread_id: ThreadId::new().to_string(),
+        agent_path: "/root/researcher".to_string(),
+    };
+
+    chat.handle_server_notification(
+        ServerNotification::ItemStarted(ItemStartedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            started_at_ms: 0,
+            item: activity.clone(),
+        }),
+        /*replay_kind*/ None,
+    );
+    chat.handle_server_notification(
+        ServerNotification::ItemCompleted(ItemCompletedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            completed_at_ms: 0,
+            item: activity,
+        }),
+        /*replay_kind*/ None,
+    );
+
+    let cells = drain_insert_history(&mut rx);
+    assert_eq!(cells.len(), 1);
+    let rendered = lines_to_single_string(&cells[0]);
+    assert_chatwidget_snapshot!("app_server_sub_agent_activity_renders_once", rendered);
+}
+
 #[tokio::test]
 async fn live_app_server_collab_wait_items_render_history() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;