Refresh the subagent picker in the background (#35693)

## Why

Opening the subagent picker could wait on thread metadata and live event-store locks, delaying terminal input. Its cached entries could also omit descendants that were not observed in the current TUI session.

## What changed

- Render the picker immediately from cached navigation state and refresh root descendants asynchronously with `thread/list`.
- Merge discovered descendants into an open picker while preserving its selection and live status, and coalesce concurrent refresh requests.
- Ignore refresh responses from a previous session and avoid blocking on busy event stores.

## Testing

- Extend session lifecycle coverage for nonblocking picker opens, refresh coalescing, descendant discovery, selection preservation, and status updates.
- Add coverage for rejecting a refresh response after navigation state is cleared.

GitOrigin-RevId: 7eae7f696f8d2d323a47b3fd2800ea1c1cdbf2a0

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

@@ -486,6 +486,11 @@ impl ListSelectionView {
         let previously_selected = self
             .selected_actual_idx()
             .filter(|actual_idx| self.enabled_actual_idx(*actual_idx).is_some())
+            .or_else(|| {
+                self.initial_selected_idx
+                    .take()
+                    .filter(|actual_idx| self.enabled_actual_idx(*actual_idx).is_some())
+            })
             .or_else(|| {
                 (!self.is_searchable)
                     .then(|| {
@@ -494,11 +499,6 @@ impl ListSelectionView {
                             .position(|item| item.is_current && Self::item_is_enabled(item))
                     })
                     .flatten()
-            })
-            .or_else(|| {
-                self.initial_selected_idx
-                    .take()
-                    .filter(|actual_idx| self.enabled_actual_idx(*actual_idx).is_some())
             });
 
         if self.is_searchable && !self.search_query.is_empty() {

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

@@ -1236,6 +1236,13 @@ impl BottomPane {
             .and_then(|view| view.selected_index())
     }
 
+    pub(crate) fn selected_index_for_present_view(&self, view_id: &'static str) -> Option<usize> {
+        self.view_stack
+            .iter()
+            .rfind(|view| view.view_id() == Some(view_id))
+            .and_then(|view| view.selected_index())
+    }
+
     pub(crate) fn active_tab_id_for_active_view(&self, view_id: &'static str) -> Option<&str> {
         self.view_stack
             .last()