Based on #3570717: Make configuration schema fully validatable.
Problem/Motivation
As a result of the change of the minimum supported core version to 10.3 in parent issue #3590369: Support for Drupal 12, it should now be possible to implement full validation of all of modules' configuration schema, introduced with change record: Stricter validation for config schema types is available, since version 10.3.0.
See the reports from Config Inspector for the System Branding block on Olivero:

Fix third_party_settings.block_class.classes:

Fix validation of:
third_party_settings.block_class.classesstring value with a validation rule.
Steps to reproduce
Proposed resolution
Fix the reported schema validation errors by adding validation constraints and the FullyValidatable property.
Examples of similar issues:
- #3379725: Make Block config entities fully validatable
- #3493946: Make configuration schema more validate-able
- #3508743: Fix schema errors and make the schema validatable
- #3508150: [META] Implement complete coverage of validatable config schema across plugins and entities
(very good description of the API and data model changes)
Remaining tasks
User interface changes
API changes
Data model changes
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Issue fork block_class-3605794
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Comments
Comment #4
dydave commented90% of the changes in this merge request were generated with AI:
Mostly: Added a Constraint Validator on CSS Classes used for configuration validation and at the block config form level (user facing validation).
The rest of the changes is all tests: Unit, Kernel, Functional.
The changes were merged above at #3, marking as Fixed , for now.
Thanks!