test(extension): add diagnostics smoke coverage

[MayerTim]

Summary

• Add a VS Code extension-host diagnostics smoke test for SQLovely missing-metadata diagnostics.
• Activate SQLovely, open a temporary .sql document, and verify the missing metadata warning is published through vscode.languages.getDiagnostics.
• Add the diagnostics smoke test to the existing extension-host suite alongside the formatting smoke test.
• Add a project validation guard for the new extension-host diagnostics test asset.
• Keep this branch test-only with no formatter, metadata, diagnostics behavior, quick-fix, tokenizer, packaging, or public documentation changes.

Validation

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