DiscardLayoutChangesForm/RevertOverridesForm::getDescription do not handle NULL context values

Problem/Motivation

When using Layout Library with Layout Builder in Drupal 11, clicking
“Discard changes” on a saved layout library item can cause a fatal error:
Call to a member function label() on null in DiscardLayoutChangesForm::getDescription().
The form expects an entity context, but SectionStorage::getContextValue('entity')
can return NULL without throwing a ContextException, and this case is not handled.

Steps to reproduce

1. Enable Layout Builder and Layout Library.
2. Create a content type with Layout Builder enabled (for example: themag_landing_page).
3. Create and save a layout library entry for that content type
   (for example: mg_home_page_6 under
   /admin/structure/types/manage/themag_landing_page/layout-library).
4. Edit the saved layout library item so that there are unsaved changes.
5. Click the “Discard changes” link, which hits a URL like:
   
   /admin/structure/types/manage/themag_landing_page/layout-library/mg_home_page_6/discard-changes.

Proposed resolution

Make DiscardLayoutChangesForm::getDescription() robust when the entity context is
missing or NULL. Instead of always calling $entity->label(), first check that
$entity is not NULL, and fall back to the generic message if there is no entity.

public function getDescription() {
  try {
    $entity = $this->sectionStorage->getContextValue('entity');
    if ($entity && $entity->label()) {
      return $this->t('Any unsaved changes to the layout for %label will be discarded. This action cannot be undone.', [
        '%label' => $entity->label(),
      ]);
    }
  }
  catch (\Drupal\Component\Plugin\Exception\ContextException $e) {
    // Fall through to the generic message below.
  }

  // If the entity context is not available or NULL, return a generic message.
  return $this->t('Any unsaved changes to the layout will be discarded. This action cannot be undone.');
}

Remaining tasks

• Add automated test coverage for the discard changes route when no entity context is available.
• Verify the fix on a real site using Layout Library and Layout Builder.

User interface changes

• No visual UI changes. Only prevents a fatal error and ensures the confirmation page always renders with either the entity label or the generic message.

Introduced terminology

• None.

API changes

• None. This only hardens an existing form method against NULL context values.

Data model changes

• None.

Release notes snippet

Fixed a fatal error when discarding unsaved changes from the Layout Builder discard form in some Layout Library scenarios, by handling missing entity context in DiscardLayoutChangesForm::getDescription().