Add first set of extension tests

[juliasilge]

This PR adds the first automated test suite for the extension ggsql-vscode, plus a GitHub Actions workflow to run it. It has 20 tests across four files, 27 TextMate scope assertions, and a Testing section in ggsql-vscode/CLAUDE.md.

The tests depend on one of the fixes from #502 FYI.

The extension as stock VS Code sees it:

Area	Tests
Activation, language resolution, command gating	4
parseCells cell splitting	7
.sql attachment gating (isGgsqlDocument)	5
CodeLens placement	4
TextMate scopes	27 assertions

This does not include anything that needs a Positron host (the language runtime manager, the Connections pane drivers, and cell execution. Also sqlAssociation.ts, manager.ts and connections.ts). I'll add that in a second step after our next release, when we'll have Linux builds for test runners.

I checked every test by breaking the code it covers and confirming it goes red, not just by watching the suite go green. A whole-suite scrutiny pass then re-ran independent mutations across all 20. IMO that kind of thing is worth doing, especially with AI-generated code and tests, which can be really low value sometimes.

Findings about the extension itself

Two things surfaced that are not test problems:

• The comment in cellParser.ts claimed a marker-free document is treated as one cell. It is not; parseCells returns no cells at all, and the whole-document fallback lives in extension.ts. Comment corrected in its own commit.
• sqlFilesEnabled()'s true fallback argument is unreachable. package.json already declares a default for ggsql.enableSqlFiles, and VS Code resolves .get() against the registered schema default first. Documented in place for now, no behaviour change.

Test infrastructure

Two runners behind npm test:

• @vscode/test-cli drives a real VS Code instance for anything needing the vscode API. CI wraps it in xvfb-run.
• vscode-tmgrammar-test checks TextMate scopes in plain Node, so npm run test:grammar is a roughly one second loop with no Electron.

Tests compile to out-test/ via a dedicated tsconfig.test.json, deliberately not to out/, which esbuild.js owns. Emitting both to out/ would have meant the test build overwriting the extension bundle.

The Mocha timeout is set to 5s, matching what I have found works well for the Quarto extension's suite.

npm run lint, npm run check-types and npm test are clean, with 20 passing plus the grammar fixture. A clean rm -rf node_modules && npm ci && npm test also passes, which is the closest local approximation of the CI run.

[thomasp85]

THANKS!