Issues/3419186: Add hook_ENTITY_TYPE_form_mode_alter().

Closes #3419186

core/modules/system/tests/modules/entity_test/entity_test.module

@@ -49,3 +49,12 @@ function entity_test_delete_bundle($bundle, $entity_type = 'entity_test'): void
   @trigger_error(__FUNCTION__ . '() is deprecated in drupal:11.2.0 and is removed from drupal:12.0.0. Use \Drupal\entity_test\EntityTestHelper::deleteBundle() instead. See https://www.drupal.org/node/3497049', E_USER_DEPRECATED);
   EntityTestHelper::deleteBundle($bundle, $entity_type);
 }
+
+/**
+ * Implements hook_ENTITY_TYPE_form_mode_alter().
+ */
+function entity_test_entity_test_form_mode_alter(&$form_mode, EntityInterface $entity): void {
+  if ($entity->getEntityTypeId() === 'entity_test' && $entity->get('name')->value === 'test_entity_type_form_mode_alter') {
+    $form_mode = 'compact';
+  }
+}