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Drupal\config\Tests\ConfigSchemaTestBase has a fabulous testing facility to ensure config schema completeness in its checkValue() method. This would be amazing to be reused as a debugging tool. Eg.could use this code to summarize no/full/partial schema validity for configuration as well as collect data granular enough to help with debugging and solving config schema issues much faster.
Abstract the logic from checkValue() out so that contrib can use this. Instead of directly throwing failures from it, collect the errors and let the implementor handle them as needed.
Agree on approach.
User interface changes
No. API addition with new schema testing functionality instead.