Initialize questions in buffered replay test messages (#42356)

## What changed

Set `questions` to `None` when the buffered replay tests construct an
ordinary completed agent message.

GitOrigin-RevId: 8b5ff14ff8ef25f4f760441767cfaed8542f8b94

codex-rs/tui/src/app/tests/buffered_replay.rs

@@ -24,6 +24,7 @@ fn completed(thread: &str) -> ServerNotification {
             phase: None,
             memory_citation: None,
             delivery: None,
+            questions: None,
         },
     })
 }