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/Breadcrumb/SectionBreadcrumbBuilder.php

@@ -6,12 +6,12 @@ namespace Drupal\site_structure\Breadcrumb;
 
 use Drupal\Core\Breadcrumb\Breadcrumb;
 use Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface;
-use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Link;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
 use Drupal\node\NodeInterface;
+use Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerManagerInterface;
 
 /**
  * Provides a breadcrumb builder for section_page and content_page content types.
@@ -27,6 +27,13 @@ class SectionBreadcrumbBuilder implements BreadcrumbBuilderInterface {
    */
   protected EntityTypeManagerInterface $entityTypeManager;
 
+  /**
+   * The parent entity handler manager.
+   *
+   * @var \Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerManagerInterface
+   */
+  protected ParentEntityHandlerManagerInterface $handlerManager;
+
   /**
    * Content types that this builder applies to.
    *
@@ -39,9 +46,15 @@ class SectionBreadcrumbBuilder implements BreadcrumbBuilderInterface {
    *
    * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
    *   The entity type manager.
+   * @param \Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerManagerInterface $handler_manager
+   *   The parent entity handler manager.
    */
-  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
+  public function __construct(
+    EntityTypeManagerInterface $entity_type_manager,
+    ParentEntityHandlerManagerInterface $handler_manager,
+  ) {
     $this->entityTypeManager = $entity_type_manager;
+    $this->handlerManager = $handler_manager;
   }
 
   /**
@@ -79,25 +92,8 @@ class SectionBreadcrumbBuilder implements BreadcrumbBuilderInterface {
     $context = $this->getParentContext($node);
 
     if ($context) {
-      switch ($context['type']) {
-        case 'user':
-          // User context: Home > User Name > ... > Current Page.
-          $this->addUserBreadcrumb($breadcrumb, $context['entity']);
-          break;
-
-        case 'group':
-          // Group context: Home > Group Name > ... > Current Page.
-          $this->addGroupBreadcrumb($breadcrumb, $context['entity']);
-          break;
-
-        default:
-          // Node or taxonomy context: use site section.
-          if ($node->hasField('field_site_section') && !$node->get('field_site_section')->isEmpty()) {
-            $term_id = $node->get('field_site_section')->target_id;
-            $this->addTaxonomyBreadcrumbs($breadcrumb, $term_id);
-          }
-          break;
-      }
+      // Use the handler manager to build breadcrumbs for the context entity.
+      $this->handlerManager->buildBreadcrumbForEntity($breadcrumb, $context['entity']);
     }
     else {
       // No parent context, check for site section directly.
@@ -193,32 +189,6 @@ class SectionBreadcrumbBuilder implements BreadcrumbBuilderInterface {
     return NULL;
   }
 
-  /**
-   * Adds user breadcrumb.
-   *
-   * @param \Drupal\Core\Breadcrumb\Breadcrumb $breadcrumb
-   *   The breadcrumb object.
-   * @param \Drupal\Core\Entity\EntityInterface $user
-   *   The user entity.
-   */
-  protected function addUserBreadcrumb(Breadcrumb $breadcrumb, EntityInterface $user): void {
-    $breadcrumb->addCacheableDependency($user);
-    $breadcrumb->addLink($user->toLink());
-  }
-
-  /**
-   * Adds group breadcrumb.
-   *
-   * @param \Drupal\Core\Breadcrumb\Breadcrumb $breadcrumb
-   *   The breadcrumb object.
-   * @param \Drupal\Core\Entity\EntityInterface $group
-   *   The group entity.
-   */
-  protected function addGroupBreadcrumb(Breadcrumb $breadcrumb, EntityInterface $group): void {
-    $breadcrumb->addCacheableDependency($group);
-    $breadcrumb->addLink($group->toLink());
-  }
-
   /**
    * Adds breadcrumbs based on taxonomy hierarchy.
    *