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.rs

@@ -334,6 +334,7 @@ use crate::status_indicator_widget::StatusDetailsCapitalization;
 use crate::text_formatting::truncate_text;
 use crate::tui::FrameRequester;
 mod command_lifecycle;
+mod connector_mentions;
 mod connectors;
 mod constructor;
 pub(crate) use self::connectors::ConnectorScopeGeneration;