Module testing

Public ./testing entry point.

Prefer the highest-level seam that reaches the code under test:

• createTestHost for framework modules and their production plan;
• a focused port fake for one capability or service;
• createVSCodeMock only for code that directly imports vscode.

This package never imports vscode at runtime. The low-level mock uses only import type, so ordinary unit tests do not require an Extension Host. That convenience is not a compatibility guarantee: platform rendering, host scheduling and unimplemented VS Code members still belong in Extension Host tests.