Show loading state when @ search results are pending (#3061)

## Summary
- allow selection popups to specify their empty state message
- show a "loading..." placeholder in the file search popup while matches
are pending
- update other popup call sites to continue using a "no matches" message

## Testing
- just fmt
- just fix -p codex-tui
- cargo test -p codex-tui

------
https://chatgpt.com/codex/tasks/task_i_68b73e956e90832caf4d04a75fcc9c46

codex-rs/tui/src/bottom_pane/command_popup.rs

@@ -195,7 +195,15 @@ impl WidgetRef for CommandPopup {
                 })
                 .collect()
         };
-        render_rows(area, buf, &rows_all, &self.state, MAX_POPUP_ROWS, false);
+        render_rows(
+            area,
+            buf,
+            &rows_all,
+            &self.state,
+            MAX_POPUP_ROWS,
+            false,
+            "no matches",
+        );
     }
 }