test(formatter): cover corpus helper safeguards

[MayerTim]

Summary

• extract formatter corpus runner utilities into a shared test helper
• keep the corpus runner focused on registering normal and CRLF fixture checks
• add focused tests for fixture pairing failures, unsupported corpus SQL names, and CRLF normalization
• keep formatter behavior and corpus SQL fixtures unchanged

Validation

• npm run validate
• npm run package:vsix