feat(tui): default reasoning selection to medium (#7040)

## Summary
- allow selection popups to request an initial highlighted row
- begin the /models reasoning selector focused on the default effort

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



https://github.com/user-attachments/assets/b322aeb1-e8f3-4578-92f7-5c2fa5ee4c98



------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_691f75e8fc188322a910fbe2138666ef)

codex-rs/tui/src/chatwidget.rs

@@ -2120,6 +2120,14 @@ impl ChatWidget {
         } else {
             default_choice
         };
+        let selection_choice = highlight_choice.or(default_choice);
+        let initial_selected_idx = choices
+            .iter()
+            .position(|choice| choice.stored == selection_choice)
+            .or_else(|| {
+                selection_choice
+                    .and_then(|effort| choices.iter().position(|choice| choice.display == effort))
+            });
         let mut items: Vec<SelectionItem> = Vec::new();
         for choice in choices.iter() {
             let effort = choice.display;
@@ -2196,6 +2204,7 @@ impl ChatWidget {
             header: Box::new(header),
             footer_hint: Some(standard_popup_hint_line()),
             items,
+            initial_selected_idx,
             ..Default::default()
         });
     }