wrap markdown at render time (#4506)

This results in correctly indenting list items with long lines.

<img width="1006" height="251" alt="Screenshot 2025-09-30 at 10 00
48 AM"
src="https://github.com/user-attachments/assets/0a076cf6-ca3c-4efb-b3af-dc07617cdb6f"
/>

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

@@ -341,6 +341,7 @@ fn make_chatwidget_manual() -> (
         ghost_snapshots: Vec::new(),
         ghost_snapshots_disabled: false,
         needs_final_message_separator: false,
+        last_rendered_width: std::cell::Cell::new(None),
     };
     (widget, rx, op_rx)
 }