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/Plugin/ParentEntityHandler/TaxonomyTermHandler.php

@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\site_structure\Plugin\ParentEntityHandler;
+
+use Drupal\Core\Breadcrumb\Breadcrumb;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\site_structure\Attribute\ParentEntityHandler;
+use Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerBase;
+use Drupal\taxonomy\TermInterface;
+
+/**
+ * Handler for taxonomy term entities.
+ */
+#[ParentEntityHandler(
+  id: 'taxonomy_term',
+  label: new TranslatableMarkup('Taxonomy Vocabularies (taxonomy_term)'),
+  entity_type_id: 'taxonomy_term',
+  clears_site_section: FALSE,
+  sort_field: 'name',
+  bundle_restrictions: ['site_section'],
+  weight: 0,
+)]
+class TaxonomyTermHandler extends ParentEntityHandlerBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getEntityFromRoute(\Drupal\Core\Routing\RouteMatchInterface $route_match): ?EntityInterface {
+    $term = $route_match->getParameter('taxonomy_term');
+
+    if (!$term instanceof TermInterface) {
+      return NULL;
+    }
+
+    if (!$this->handlesEntity($term)) {
+      return NULL;
+    }
+
+    return $term;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildBreadcrumb(Breadcrumb $breadcrumb, EntityInterface $entity): void {
+    if (!$entity instanceof TermInterface) {
+      return;
+    }
+
+    // Add all ancestors in the taxonomy hierarchy.
+    $term_storage = $this->entityTypeManager->getStorage('taxonomy_term');
+    $ancestors = $term_storage->loadAllParents($entity->id());
+    $ancestors = array_reverse($ancestors);
+
+    foreach ($ancestors as $ancestor) {
+      $breadcrumb->addCacheableDependency($ancestor);
+      $breadcrumb->addLink($ancestor->toLink());
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSiteSectionId(EntityInterface $entity): int|string|null {
+    if (!$entity instanceof TermInterface) {
+      return NULL;
+    }
+
+    // Only site_section terms can be site sections.
+    if ($entity->bundle() !== 'site_section') {
+      return NULL;
+    }
+
+    return $entity->id();
+  }
+
+}