test(extension): add formatting smoke coverage

[MayerTim]

Summary

• Add explicit VS Code extension-host smoke-test infrastructure with @vscode/test-electron.
• Add npm run test:extension for running extension-host tests separately from the fast default validation suite.
• Add one formatting smoke test that activates SQLovely, opens a temporary .sql document, requests edits through VS Code's document-formatting provider command, applies the edits, and verifies the formatted output.
• Add project validation guards for the extension-host test assets and .vscode-test local-download hygiene.
• Keep this branch test-only with no formatter implementation, metadata logic, diagnostics, extension behavior, or formatter default changes.

Validation

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

Notes

This PR adds the single remaining standalone test layer after the formatter corpus hardening phase: a minimal VS Code extension-host formatting smoke test. It intentionally does not broaden into diagnostics, quick fixes, metadata behavior, or a configuration matrix.