Issue #3580070: Reject invalid default block configs

I reverted the broader changes and reworked this to stay within the review feedback.

The fix now happens in \Drupal\canvas\Plugin\Canvas\ComponentSource\BlockComponentDiscovery::checkRequirements(): block plugins are rejected during discovery if their defaultConfiguration() already violates their own config schema. That keeps invalid/lying block plugins from getting generated as Canvas components in the first place.

I also replaced the previous preview/API-oriented approach with a focused kernel test that covers this discovery-time failure case.

One compatibility detail remains in the diff: the BlockManagerDecorator method signature update is kept because it is required for the current core interface on this branch.