Move the headless smoke test into the repo #131

Why: a working smoke test exists outside the repo (it opens a failing Java file in headless VS Code, runs LiquidJava: Verify, and checks for the refinement error). It checks the main feature end to end but only runs by hand on one machine.

What:

• Add @vscode/test-cli, @vscode/test-electron and a .vscode-test.mjs.
• Put the fixture in client/test-fixtures/failing/src/main/java/. It must be outside client/src because of the src path rule in JUnit tests for the language server #129, and each fixture needs its own workspace because the server only reads the first workspace folder.
• Test that the extension activates, ready resolves, and Verify produces the expected Refinement Error diagnostic (waiting on onDiagnostics, not sleeping).
• Add an integration job to test.yml, running on every branch push like the rest (one fixture on stable should take a few minutes):
  • setup-java pinned to 21, with JAVA_HOME set (the client checks JAVA_HOME first, and the server needs Java 20 or newer)
  • xvfb-run -a npm test
  • a 120s Mocha timeout for the first verify (cold JVM, Spoon and z3)
  • VS Code logs uploaded as an artifact on failure

References:

• Official sample config: https://github.com/microsoft/vscode-extension-samples/blob/main/helloworld-test-cli-sample/.vscode-test.mjs
• vscode-go smoke tests on PRs with xvfb-run -a npm run test: https://github.com/golang/vscode-go/blob/master/.github/workflows/test-smoke.yml
• vscode-java integration tests against Maven/Gradle fixture projects: https://github.com/redhat-developer/vscode-java/tree/main/test/resources/projects