ci: fix build and tests - #1731

Summary

• update @vscode/test-electron to 3.1.0 so current macOS bundles are launched through CFBundleExecutable
• run Xvfb only on Linux and run tests directly on macOS and Windows
• move CI to Node.js 24 and Node 24-based v5 official actions
• remove the obsolete programmatic test runner left after the vscode-test CLI migration

Root cause

VS Code 1.110 renamed the macOS executable from Electron to Code.
The compatibility symlink was subsequently removed, while @vscode/test-electron 2.5.2 continued to launch Contents/MacOS/Electron.

As a result, VS Code 1.138 downloaded successfully on macOS ARM64, but failed with ENOENT before the Mocha integration suite started.

@vscode/test-electron 3.1.0 resolves the executable using the app bundle's CFBundleExecutable, while retaining compatibility with older bundles.