Tests for the webview script (Vitest + jsdom) #128

Why: tab switching, the file/workspace toggle, and clicks that open a file or highlight a variable all live in client/src/webview/script.ts, and nothing checks them today. getScript(vscode, document, window) already takes its dependencies as arguments, so no VS Code API needs faking.

What:

• Call getScript with a { postMessage: vi.fn() } spy and a jsdom document and window.
• Send messages with window.dispatchEvent(new MessageEvent('message', { data })), then check the rendered DOM and the spy's calls.
• Stub window.mermaid for the state machine view, and reset modules between tests (diagram.ts keeps zoom/pan state at module level).
• First cases: the ready message, tab switching, show-all toggling, and openFile/highlight messages on click.

References:

• vscode-pull-request-github tests its webview UI with jsdom: https://github.com/microsoft/vscode-pull-request-github/blob/main/webviews/editorWebview/test/app.test.tsx