[codex] Avoid forced directory refresh during plugin install auth checks (#25381)

## Summary
- Use normal directory loading for plugin install app metadata so
install avoids forced directory refresh while still loading metadata on
cold cache.
- Continue force-refreshing codex_apps tools for auth state.
- Add regression coverage that pre-warms the directory cache and asserts
install returns cached app metadata without extra directory requests.

## Validation
- just fmt
- git diff --check
- just test -p codex-app-server plugin_install_returns_apps_needing_auth
plugin_install_filters_disallowed_apps_needing_auth (blocked locally:
cargo-nextest is not installed)