Implement ParentEntityHandler plugin system for extensible entity support

Replace hardcoded entity type checks with a plugin-based architecture using
PHP 8 attributes. This allows adding new parent entity types without modifying
core module files.

Changes:
- Add ParentEntityHandler attribute, interface, base class, and manager
- Create built-in handlers for taxonomy_term, user, and node entities
- Move Group support to site_structure_group submodule (fixes class not found
  error when Group module is not installed)
- Refactor SiteStructureSettingsForm to use handler manager
- Refactor SiteStructureMenuBlock to use handler manager
- Refactor SectionBreadcrumbBuilder to use handler manager
- Update site_structure.module to use handler manager for clearsSiteSection
- Update documentation and translations

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Form/SiteStructureSettingsForm.php

@@ -7,9 +7,9 @@ namespace Drupal\site_structure\Form;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
-use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerManagerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -32,23 +32,11 @@ class SiteStructureSettingsForm extends ConfigFormBase {
   protected EntityTypeBundleInfoInterface $entityTypeBundleInfo;
 
   /**
-   * The module handler.
+   * The parent entity handler manager.
    *
-   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   * @var \Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerManagerInterface
    */
-  protected ModuleHandlerInterface $moduleHandler;
-
-  /**
-   * Entity types that can be used as parents.
-   *
-   * @var array
-   */
-  protected array $supportedEntityTypes = [
-    'node' => 'Content Types (node)',
-    'taxonomy_term' => 'Taxonomy Vocabularies (taxonomy_term)',
-    'user' => 'Users (user)',
-    'group' => 'Groups (group)',
-  ];
+  protected ParentEntityHandlerManagerInterface $handlerManager;
 
   /**
    * Constructs a SiteStructureSettingsForm object.
@@ -59,19 +47,19 @@ class SiteStructureSettingsForm extends ConfigFormBase {
    *   The entity type manager.
    * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
    *   The entity type bundle info service.
-   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
-   *   The module handler.
+   * @param \Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerManagerInterface $handler_manager
+   *   The parent entity handler manager.
    */
   public function __construct(
     ConfigFactoryInterface $config_factory,
     EntityTypeManagerInterface $entity_type_manager,
     EntityTypeBundleInfoInterface $entity_type_bundle_info,
-    ModuleHandlerInterface $module_handler,
+    ParentEntityHandlerManagerInterface $handler_manager,
   ) {
     parent::__construct($config_factory);
     $this->entityTypeManager = $entity_type_manager;
     $this->entityTypeBundleInfo = $entity_type_bundle_info;
-    $this->moduleHandler = $module_handler;
+    $this->handlerManager = $handler_manager;
   }
 
   /**
@@ -82,7 +70,7 @@ class SiteStructureSettingsForm extends ConfigFormBase {
       $container->get('config.factory'),
       $container->get('entity_type.manager'),
       $container->get('entity_type.bundle.info'),
-      $container->get('module_handler'),
+      $container->get('plugin.manager.parent_entity_handler'),
     );
   }
 
@@ -137,20 +125,12 @@ class SiteStructureSettingsForm extends ConfigFormBase {
       '#tree' => TRUE,
     ];
 
-    foreach ($this->supportedEntityTypes as $entity_type => $label) {
-      // Check if entity type exists.
-      if (!$this->entityTypeManager->hasDefinition($entity_type)) {
-        continue;
-      }
-
-      // Special handling for 'group' - check if module is enabled.
-      if ($entity_type === 'group' && !$this->moduleHandler->moduleExists('group')) {
-        continue;
-      }
+    $supportedEntityTypes = $this->handlerManager->getSupportedEntityTypes();
 
+    foreach ($supportedEntityTypes as $entity_type => $label) {
       $form['allowed_parent_targets'][$entity_type] = [
         '#type' => 'details',
-        '#title' => $this->t($label),
+        '#title' => $label,
         '#open' => TRUE,
       ];
 
@@ -207,8 +187,9 @@ class SiteStructureSettingsForm extends ConfigFormBase {
   public function submitForm(array &$form, FormStateInterface $form_state): void {
     $targets = [];
     $values = $form_state->getValue('allowed_parent_targets');
+    $supportedEntityTypes = $this->handlerManager->getSupportedEntityTypes();
 
-    foreach ($this->supportedEntityTypes as $entity_type => $label) {
+    foreach ($supportedEntityTypes as $entity_type => $label) {
       if (empty($values[$entity_type])) {
         continue;
       }
@@ -276,12 +257,7 @@ class SiteStructureSettingsForm extends ConfigFormBase {
       }
 
       if (!isset($entity_type_settings[$entity_type])) {
-        $sort_field = match($entity_type) {
-          'taxonomy_term' => 'name',
-          'user' => 'name',
-          'group' => 'label',
-          default => 'title',
-        };
+        $sort_field = $this->handlerManager->getSortField($entity_type);
 
         $entity_type_settings[$entity_type] = [
           'handler' => 'default:' . $entity_type,