ComponentMetadataRequirementsChecker::check() should validate that the example(s) actually match the JSON schema

Overview

Broken out from #3507641: Allow components containing non-required images and no examples to be placed

\Drupal\experience_builder\ComponentMetadataRequirementsChecker::check() only checks for the presence of an examples[0], but it does not verify that it actually complies with the JSON Schema!

… because you'd reasonably expect that JSON Schema validates its examples, but … nope:

This keyword can be used to provide sample JSON values associated with a particular schema, for the purpose of illustrating usage. It is RECOMMENDED that these values be valid against the associated schema.

— https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-validati... 🧟‍♀️😭

So it's up to either Drupal core's SDC subsystem to strictly validate this (it doesn't), or up to XB.

Conclusion: XB must do this, since for XB it is critical.

(For those of you wondering: well, what if we'd use default instead? — ignoring the fact that the SDC subsystem doesn't use this at all, the same "recommended to be valid" statement is present there: https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-validati... 😬)

Proposed resolution

1. Add an SDC to the xb_test_sdc module with invalid example
2. Update SingleDirectoryComponent::checkRequirements() to detect and disallow this
3. Update \Drupal\Tests\experience_builder\Kernel\Plugin\ExperienceBuilder\ComponentSource\SingleDirectoryComponentTest::testDiscovery() expectations

User interface changes

None.