Test rich text props in page variant config translation

Adds regression coverage for the WSOD reported in #3591964: translating a page variant (e.g. the Haven footer) crashed when it contained a rich text prop, because CKEditor 5's widget alter needs a host entity.

That host entity is already provided on 1.x (CanvasStaticPropSourceFieldWidget::buildWidgetForm()), so this MR only locks it in with a test. ConfigWithComponentTreeConfigTranslationUiTest now adds a rich text prop to the page variant and checks that:

• the translation form renders its value field and canvas_html_block format,
• the saved French override contains the translated HTML.

Testing instructions

• phpunit tests/src/Functional/ConfigWithComponentTreeConfigTranslationUiTest.php passes
• Manual: Drupal CMS + Haven, enable config translation, add a language, translate the Footer page region. The form loads with a CKEditor field.

Closes #3591964