Fix macOS test launch with VS Code 1.131 (#1684)

### Why

VS Code 1.131 removed the legacy `Contents/MacOS/Electron` compatibility
path from its macOS application bundle. `@vscode/test-electron` 2.5.2
still launches that hard-coded path, so all macOS smoke jobs fail with
`ENOENT` before VS Code or the extension starts.

This currently blocks the macOS checks on #1656. The upstream issue and
fix are microsoft/vscode-test#349 and microsoft/vscode-test#350.

### What

Upgrade `@vscode/test-electron` from 2.5.2 to 3.1.0 and refresh the
lockfile. Version 3.1.0 resolves the macOS executable from
`CFBundleExecutable` in `Info.plist`, with the old `Electron` path
retained as a fallback for older VS Code builds.

This is a development-only dependency change. Version 3.1.0 requires
Node 22, which is already used by CI.

### Validation

- `npm run compile-tests`
- `npm run unittest` — 1469 passing, 0 failing, 5 pending

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 39dcc6a3-0fbd-4f36-9d0f-68677de49c27

Author: StellaHuang95

package-lock.json

@@ -723,7 +723,7 @@
         "@types/which": "^3.0.4",
         "@typescript-eslint/eslint-plugin": "^8.16.0",
         "@typescript-eslint/parser": "^8.16.0",
-        "@vscode/test-electron": "^2.5.2",
+        "@vscode/test-electron": "^3.1.0",
         "@vscode/vsce": "^3.9.2",
         "eslint": "^9.15.0",
         "glob": "^8.1.0",