test(formatter): add statement continuation corpus guards

[MayerTim]

Summary

• Add a Watcom corpus fixture for compact UPDATE ... SET assignment continuations and arithmetic spacing.
• Add a Watcom corpus fixture for compact select-list comma spacing while preserving commas inside strings and quoted identifiers.
• Preserve current formatter behavior with expected-output and idempotency checks.
• Keep this branch test-only with no extension behavior or formatter default changes.

Validation

• npm run test:corpus
• npm run validate
• npm run package:vsix

Notes

This PR fills a small corpus coverage gap identified by the private coverage inventory. It does not change formatter implementation, formatter defaults, extension behavior, product behavior, or public documentation.