Add structural_pages_event submodule for Event entity support

Bridge submodule that registers Event entities (from event_management
module) as valid parent types for content_page nodes. Events act as
context containers (clears_site_section: TRUE), similar to users and
groups.

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

modules/structural_pages_event/src/Plugin/ParentEntityHandler/EventHandler.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\structural_pages_event\Plugin\ParentEntityHandler;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\event_management\Entity\EventInterface;
+use Drupal\structural_pages\Attribute\ParentEntityHandler;
+use Drupal\structural_pages\ParentEntityHandler\ParentEntityHandlerBase;
+
+/**
+ * Handler for event entities.
+ *
+ * This handler is provided by the structural_pages_event submodule and is only
+ * available when the Event Management module is installed.
+ */
+#[ParentEntityHandler(
+  id: 'event',
+  label: new TranslatableMarkup('Events (event)'),
+  entity_type_id: 'event',
+  provider_module: 'event_management',
+  clears_site_section: TRUE,
+  sort_field: 'title',
+  weight: 20,
+)]
+class EventHandler extends ParentEntityHandlerBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getEntityFromRoute(RouteMatchInterface $route_match): ?EntityInterface {
+    $event = $route_match->getParameter('event');
+
+    if (!$event instanceof EventInterface) {
+      return NULL;
+    }
+
+    return $event;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function handlesEntity(EntityInterface $entity): bool {
+    return $entity instanceof EventInterface;
+  }
+
+}