Stabilize the realtime steering timeline test (#52923)

The initial user item seals realtime transcript segments, so starting speech before it completes can split the segment under test. Wait for its `item/completed` notification and assert that it is a `ThreadItem::UserMessage` before triggering speech, so only the steering under test can split that segment.

GitOrigin-RevId: 932443f4a0d6c4284cf95f44e64912506769a180

codex-rs/app-server/tests/suite/v2/realtime_conversation.rs

@@ -1273,6 +1273,16 @@ async fn realtime_timeline_splits_accepted_steering_and_persists_promoted_artifa
         .read_notification::<TurnStartedNotification>("turn/started")
         .await?;
 
+    // The initial user item also seals realtime transcript segments. Wait for
+    // it before speaking so only the steering under test can split this segment.
+    let initial_message = harness
+        .read_notification::<ItemCompletedNotification>("item/completed")
+        .await?;
+    assert!(matches!(
+        initial_message.item,
+        ThreadItem::UserMessage { .. }
+    ));
+
     harness
         .append_text(harness.thread_id.clone(), "Trigger speech")
         .await?;