Make sure our UI respects the "no_ui" plugin key

Since our notification plugin type (indirectly) inherits from \Drupal\search_api\Plugin\HideablePluginBase and \Drupal\search_api\Plugin\HideablePluginInterface, it defines 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 notification plugin type Active will explicitly document the existence of this property.