KAFKA-18967: Add schema checker evolution tests

[lh0156]

Summary

• Add schema evolution coverage using distinct parent and child specifications.
• Cover common-struct evolution through the MetadataSchemaCheckerTool entry point.
• Reject a field that is present in both message versions but missing from the child instead of silently accepting it.

Motivation

KAFKA-18967 asks for tests that exercise the metadata schema checker beyond comparing a schema with itself. The new cases cover the field-pair iteration and per-message struct registries involved in the KAFKA-18955 fixes. While adding the coverage, the tests exposed a remaining false positive where FieldSpecPairIterator dropped an unmatched field after avoiding the previous infinite loop.

Validation

• ./gradlew :generator:test --no-build-cache --console=plain