tui: separator above final agent message (#4324)

Adds a separator line before the final agent message

<img width="1011" height="884" alt="Screenshot 2025-09-26 at 4 55 01 PM"
src="https://github.com/user-attachments/assets/7c91adbf-6035-4578-8b88-a6921f11bcbc"
/>

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

@@ -340,6 +340,7 @@ fn make_chatwidget_manual() -> (
         is_review_mode: false,
         ghost_snapshots: Vec::new(),
         ghost_snapshots_disabled: false,
+        needs_final_message_separator: false,
     };
     (widget, rx, op_rx)
 }