Ensure placeholder blocks cannot be placed in any UI

In a recipe, you would probably add a placeholder block with a config action:

 config:
    actions:
        dashboard.dashboard.testing:
            addComponentToLayout:
                section: 0
                position: 1
                component:
                    region: first
                    configuration:
                        id: dashboard_placeholder
                        decorates: 'views_block:my_pages-block'
                        items_per_page: 6
                        more: ['configuration', 'options', 'here']

Drupal CMS's Admin UI recipe just embeds them directly: https://git.drupalcode.org/project/drupal_cms/-/blob/2.x/recipes/drupal_...

So to test it, I would just apply the config action programmatically, and skip messing with a recipe at all. Something like:

\Drupal::service('plugin.manager.config_action')
    ->applyAction('dashboard.dashboard.test', 'addComponentToLayout', [
        'section' => 0,
        'position' => 1,
        'component' => [
            'region' => 'first',
            'configuration' => [
                'id' => 'dashboard_placeholder',
                'decorates' => 'block_content:TEST_BLOCK_UUID',
                'label_display' => '0',
            ],
        ],
    ]);

Similar to how the addComponentToLayout action is tested in core: https://git.drupalcode.org/project/drupal/-/blob/11.x/core/modules/layou...

That makes sense. I think what needs to happen is that buildConfigurationForm() and related methods need to be turned into no-ops. The configuration form should probably just always show a message that says "This is a placeholder block and cannot be edited in the UI."