Non-serializable #submit callback breaks AJAX rebuilds on clone form

Environment

• Drupal: 11.3.13
• PHP: 8.5.5
• Database: MariaDB 10.11.16
• DDEV: v1.25.2
• Content templates: 1.3.0
• Quick Node Clone: enabled
• Browser: Chrome

Prerequisites

• Install and enable the module (pulls in media and quick_node_clone):
  

  ddev composer require drupal/content_templates:1.3.0
  ddev drush en content_templates -y

• An article node with a populated image field, and a content template built from it, so the clone form shows the image with a Remove button:
  

  ddev drush php-eval '
  $data = file_get_contents("core/misc/druplicon.png");
  $uri = \Drupal::service("file_system")->saveData($data, "public://druplicon_test.png", 1);
  $file = \Drupal::entityTypeManager()->getStorage("file")->create(["uri" => $uri, "status" => 1]);
  $file->save();
  $n = \Drupal::entityTypeManager()->getStorage("node")->load(4);
  $n->set("field_image", ["target_id" => $file->id(), "alt" => "Test"]);
  $n->save();
  $t = \Drupal::entityTypeManager()->getStorage("content_template")->create([
    "name" => "My template", "status" => 1, "field_source" => 4,
  ]);
  $t->save();'

  (Replace 4 with the node id used.)

Root cause

Since the 1.3.0 OOP hook conversion, the Quick Node Clone form submit handler was registered as an object callback [$this, 'quickNodeCloneSubmit']. On /template/{node}/quick_clone, AJAX interactions that rebuild and cache the form (e.g. removing an image) serialize the form array; the object callback drags in the hooks service and its dependency graph, which indirectly references the database connection, so serialization fails.

Steps

1. Apply the fix from MR !13: register the clone submit handler as a serializable static callback ([self::class, 'quickNodeCloneSubmit']) and make quickNodeCloneSubmit() a static method.
2. Rebuild caches: ddev drush cr
3. Log in as an administrator and open /template/4/quick_clone.
4. In the Image field, click Remove and observe the AJAX response.

Expected results

• The image widget rebuilds without error; the image is removed and the field returns to its empty upload state.
• No HTTP 500 and no serialization exception in the browser console or logs.

Actual results

Before the fix, clicking Remove on the image field of the clone form returned an AJAX HTTP 500 with LogicException: The database connection is not serializable (thrown while writing the rebuilt form to the form cache), because the [$this, 'quickNodeCloneSubmit'] object callback pulled the hooks service and its database-connection reference into serialization. After the fix, the callback is a serializable static reference, the form caches cleanly, and the Remove interaction rebuilds the widget without any error.

Testing produced with the assistance of an LLM.

I have used dependency serialization trait instead. As the approach was moved to use hook classes, using static methods means using static calls to `\Drupal` global object to call services, etc. This prevents the hook class from using the DI that is actually the main reason for using hook class.

Please review the updated MR.

Confirmed and tested. Reproduced the original bug on 1.3.0: cloning a node with an image field via /clone/{node}/quick_clone and removing the image via AJAX throws LogicException: The database connection is not serializable, from FormBuilder::setCache() when the form (including the ContentTemplatesHooks service reference from the submit callback) gets serialized into the form cache.

Applied MR !13 as currently pushed (DependencySerializationTrait on ContentTemplatesHooks) — the AJAX rebuild now completes without error, and the clone submission itself still works correctly afterward.

Added to merge train. Thank you everyone!