FIx: show error message when running npm install without open workspace

[ot7Deity]

Description
Fixes a silent failure where running _runNpmInstall() did nothing if a user did not have a folder open in the workspace. Added a clear error message utilizing vscode.window.showErrorMessage to provide immediate feedback to the user.

How to Test

1. Close all open folders/workspaces in VS Code.
2. Trigger the vscode-extras.runNpmInstall command (via the status bar or command palette).
3. Verify that a native VS Code error notification appears stating: "npm install requires an open workspace folder".
4. Open a workspace folder, run the command again, and verify that the terminal spawns and runs the install script normally.

[Copilot]

Pull request overview

Fixes a silent no-op in the vscode-extras selfhost extension by surfacing an explicit user-facing error when vscode-extras.runNpmInstall is invoked without any open workspace folder, improving feedback in the VS Code workbench.

Changes:

• Show an error notification and early-return when _runNpmInstall() is executed without an open workspace folder.