Use Option symbol for mac key hints (#5582)

## Summary
- show the Option (⌥) symbol in key hints when the TUI is built for
macOS so the shortcut text matches the platform terminology

## Testing
- cargo test -p codex-tui

------
https://chatgpt.com/codex/tasks/task_i_68fab7505530832992780a9e13fb707b

codex-rs/tui/src/status_indicator_widget.rs

@@ -284,6 +284,11 @@ mod tests {
         terminal
             .draw(|f| w.render_ref(f.area(), f.buffer_mut()))
             .expect("draw");
+        #[cfg(target_os = "macos")]
+        insta::with_settings!({ snapshot_suffix => "macos" }, {
+            insta::assert_snapshot!(terminal.backend());
+        });
+        #[cfg(not(target_os = "macos"))]
         insta::assert_snapshot!(terminal.backend());
     }