Use installed callable apps for TUI mentions (#39051)

## Why

The app directory is a discovery catalog and can include apps that are not
callable in the active thread. Using it for `$` suggestions can therefore
offer mentions that cannot be used.

## What changed

- Load mention candidates from `app/installed`, enrich callable apps through
  bounded `app/read` batches, and retain compatibility with app servers that
  do not support `app/installed`.
- Keep the mention catalog separate from the `/apps` directory cache and scope
  results to the current thread, workspace, and account.
- Refresh mentions after app notifications, enablement changes, configuration
  updates, and apps MCP readiness, while immediately removing disabled apps.

## Testing

Add coverage for batching and fallback behavior, stale scoped results,
notification refreshes, enablement changes, MCP startup races, and filtering
non-callable apps from mentions.

GitOrigin-RevId: 4d9e20794aed30cc97120170792fb555b86fe1e7

codex-rs/tui/src/chatwidget/session_flow.rs

@@ -148,9 +148,7 @@ impl ChatWidget {
         }
         self.transcript.saw_copy_source_this_turn = false;
         self.refresh_skills_for_current_cwd(/*force_reload*/ true);
-        if self.connectors_enabled() {
-            self.prefetch_connectors();
-        }
+        self.refresh_connector_mentions(/*force_refresh*/ false);
         self.submit_initial_user_message_if_pending();
         if display == SessionConfiguredDisplay::Normal
             && let Some(forked_from_id) = forked_from_id