Make sure our UID respects the "no_ui" plugin key

Since our two plugin types (indirectly) inherit from \Drupal\search_api\Plugin\HideablePluginBase and \Drupal\search_api\Plugin\HideablePluginInterface, they define an isHidden() method which is (by default) based on the no_ui key of the plugin definition.
However, I think we don’t actually call that method in our UI code, at the very least there is no test coverage for it. We should change that, especially since 📌 Add attribute discovery support for the search and suggester plugin types Active will explicitly document the existence of this property.