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/selection_popup_common.rs

@@ -36,11 +36,12 @@ pub(crate) fn render_rows(
     state: &ScrollState,
     max_results: usize,
     _dim_non_selected: bool,
+    empty_message: &str,
 ) {
     let mut rows: Vec<Row> = Vec::new();
     if rows_all.is_empty() {
         rows.push(Row::new(vec![Cell::from(Line::from(
-            "no matches".dim().italic(),
+            empty_message.dim().italic(),
         ))]));
     } else {
         let max_rows_from_area = area.height as usize;