Add testbed reproducing diagnostic pull on a saved unsaved-but-named buffer

[Copilot]

Saving an "unsaved but named" (untitled:) buffer makes VS Code send didSave (file URI), then didChange/didClose followed by a textDocument/diagnostic request — all on the now-closed untitled: URI. A real server has no project for that URI and the request fails. This adds a minimal testbed to reproduce the flow (#1797 / #1771).

All changes are confined to the manual testbed/ extension and do not affect the library build or CI.

Changes

• Fake language (testbed/package.json): registers a testbed language with the .testbed extension.
• Client wiring (testbed/client/src/extension.ts): adds { language: 'testbed' } to documentSelector and .testbed to the diagnosticPullOptions.match filter.
• Server (testbed/server/src/server.ts): logs didOpen/didSave/didClose and the diagnostic pull; fails the pull with no project found for URI <uri> when the target document is not open and its scheme is not file (the closed untitled: case), mirroring tsserver.
• Repro assets: testbed/workspace/sample.testbed and testbed/workspace/repro-1797.md.

Server reproduction

if (document === undefined && uri.scheme !== 'file') {
    const message = `no project found for URI ${param.textDocument.uri}`;
    connection.console.error(`error handling method 'textDocument/diagnostic': ${message}`);
    throw new ResponseError(ErrorCodes.InvalidParams, message);
}

Exercising the built server over stdio with a diagnostic pull for a closed untitled: document surfaces:

error handling method 'textDocument/diagnostic': no project found for URI untitled:/.../newFile.testbed