test(formatter): guard corpus fixture pairing

[MayerTim]

Summary

• fail corpus validation when an input fixture is missing its expected output pair
• fail corpus validation when an expected fixture has no matching input fixture
• reject unsupported .sql files under test/corpus/ so stale overlay files are not silently ignored
• keep existing formatter output and idempotency checks unchanged

Validation

• npm run validate
• npm run package:vsix