PageBlockDisplayVariant requires the concrete BlockManager class, fatal when plugin.manager.block is decorated

Problem/Motivation

PageBlockDisplayVariant::__construct() type-hints the concrete class Drupal\Core\Block\BlockManager for
$block_manager (src/Plugin/DisplayVariant/PageBlockDisplayVariant.php, line 67).

A module that decorates the plugin.manager.block service replaces it with a class that implements BlockManagerInterface
but does not extend BlockManager. Page Manager then fatals every time it instantiates a Block page variant:

TypeError: Drupal\page_manager\Plugin\DisplayVariant\PageBlockDisplayVariant::__construct():
  Argument #8 ($block_manager) must be of type Drupal\Core\Block\BlockManager,
  Drupal\canvas\Block\BlockManagerDecorator given, called in
  page_manager/src/Plugin/DisplayVariant/PageBlockDisplayVariant.php on line 77

Drupal Canvas decorates plugin.manager.block this way
(Drupal\canvas\Block\BlockManagerDecorator). Any other module that decorates the service triggers the same error.

Existing pages break, not only new ones. A Block page variant created before the decorating module was enabled fails as soon as the module
is enabled:

Site builders can't fix the affected pages through the UI, because their edit screens also fail.

The narrower type isn't needed:

• The parent class, CTools' BlockDisplayVariant::__construct(), already accepts BlockManagerInterface $block_manager.
• PageBlockDisplayVariant only passes $block_manager on to the parent. It calls no BlockManager-only
  methods.
• Page Manager already depends on the interface elsewhere, for example
  

  PageManagerController::__construct(BlockManagerInterface $block_manager,
    …)

  .

Steps to reproduce

1. Install Drupal 11 with Page Manager 8.x-4.x (page_manager_ui) and CTools 4.1.x.
2. At /admin/structure/page_manager, add a page with a Block page variant and visit the page. It returns 200.
3. Enable a module that decorates plugin.manager.block, for example Drupal Canvas 1.x.
4. Visit the page again. It returns 500 with the TypeError above.
5. Open the page's edit screen, or click Add variant → type Block page → Next. Both return
   500.

Reproduced with Drupal 11.4.8, PHP 8.4, Page Manager 8.x-4.0, CTools 4.1.1, and Drupal Canvas 1.x.

Proposed resolution

Type-hint the interface, matching the parent class:

-use Drupal\Core\Block\BlockManager;
  +use Drupal\Core\Block\BlockManagerInterface;
  …
  -   * @param \Drupal\Core\Block\BlockManager $block_manager
  +   * @param \Drupal\Core\Block\BlockManagerInterface $block_manager
  …
  -  public function __construct(…, Token $token, BlockManager $block_manager, ConditionManager $condition_manager, ModuleHandlerInterface
  $module_handler) {
  +  public function __construct(…, Token $token, BlockManagerInterface $block_manager, ConditionManager $condition_manager, ModuleHandlerInterface
  $module_handler) {

This widens the accepted type, so existing callers keep working. PHP does not apply signature-compatibility rules to constructors, so subclasses
that override the constructor are not affected.

Remaining tasks

• MR with the change above
• Test coverage: construct PageBlockDisplayVariant with a BlockManagerInterface implementation that does not extend
  BlockManager. The existing PageBlockDisplayVariantTest only prophesizes the concrete BlockManager.
• Review

API changes

PageBlockDisplayVariant::__construct() accepts BlockManagerInterface instead of BlockManager. The type is
wider, so this change is backward compatible.