Only show Worked for after the final assistant message (#7854)

Before:
<img width="1908" height="246" alt="image"
src="https://github.com/user-attachments/assets/f4d5993a-8d37-4982-a6fd-d37f449215b2"
/>
After:
<img width="1102" height="586" alt="image"
src="https://github.com/user-attachments/assets/e833140d-690a-4c33-8bc7-e2b69b9dc92d"
/>

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

@@ -442,6 +442,7 @@ fn make_chatwidget_manual(
         last_rendered_width: std::cell::Cell::new(None),
         feedback: codex_feedback::CodexFeedback::new(),
         current_rollout_path: None,
+        task_started_at: None,
     };
     (widget, rx, op_rx)
 }