[drupalMedia] Remove manual dataDowncast from DrupalMediaEditing

Problem/Motivation

DrupalMediaEditing plugin is manually downcasting the element for preview.

This is a stable blocker because this prevents arbitrary attributes from being sent to the server as part of the element that is used for rendering the preview. This could lead into inaccurate previews in the case that an arbitrary attributes are required for rendering the preview on the server side.

Proposed resolution

Improve this so that the manual step isn't needed, and the model is downcasted properly.

Remaining tasks

1. Delete
   

   /**
    * MediaFilterController::preview requires the saved element.
    * Not previewing data-caption since it does not get updated by new changes.
    * @todo: is there a better way to get the rendered dataDowncast string
    *   https://www.drupal.org/project/ckeditor5/issues/3231337?
    */
   const renderElement = (modelElement) => {
     const attrs = modelElement.getAttributes();
     let element = '<drupal-media';
     Array.from(attrs).forEach((attr) => {
       if (attr[0] !== 'data-caption') {
         element += ` ${attr[0]}="${attr[1]}"`;
       }
     });
     element += '></drupal-media>';
   
     return element;
   };
   

   ()done

2. Update
   

               this._fetchPreview(this.previewURL, {
                 text: renderElement(modelElement),
                 uuid: modelElement.getAttribute('data-entity-uuid'),
               }).then(({ label, preview }) => {
   

   . (done)

User interface changes

API changes

Data model changes