feat: add virtual workspace support via registerUriTranslator API

[jianyexi]

Summary

Add virtual workspace support via a registerUriTranslator API, following the same pattern Pylance uses.

VFS providers (e.g., Microsoft Fabric Data Engineering VS Code extension) implement a RuffUriTranslator and register it with Ruff. The translator handles bidirectional URI translation between virtual workspace URIs and local disk cache URIs, enabling Ruff's linting, formatting, and auto-fix to work on virtual files.

Closes #1007

Design

The VFS provider owns the disk cache. Ruff exports the API.

const ruff = vscode.extensions.getExtension('charliermarsh.ruff');
await ruff.activate();
ruff.exports.registerUriTranslator({
  async initialize(folders) { /* sync config files to local cache */ },
  translateToDisk(uri) { /* virtual → cached file:// */ },
  translateToVirtual(uri) { /* file:// → virtual */ },
});
// Ruff restarts server automatically with translator wired in

RuffUriTranslator interface

• initialize(workspaceFolders) — async, called once before server start. VFS provider syncs config files, creates directory structures, pre-populates translation caches.
• translateToDisk(uri) — synchronous, returns file:// URI for a virtual URI.
• translateToVirtual(uri) — synchronous, returns virtual URI for a file:// URI.

How it works

1. Ruff defers server startup in virtual workspaces until a translator registers
2. On registration, runServer() is called
3. translator.initialize() prepares the local cache
4. uriConverters on the LanguageClient translate all URIs via the translator
5. Settings (cwd, workspace) are translated before sending to the server
6. workspace/configuration middleware translates dynamically requested settings
7. Config file changes trigger workspace/didChangeWatchedFiles notifications

Changes

File	Lines	Purpose
src/common/uriTranslator.ts	+110	Interface + API + registration logic
src/common/vfsSupport.ts	+188	Translator init, uriConverters, middleware, settings translation
src/common/server.ts	+47	Calls vfsSupport functions
src/extension.ts	+48	Returns API, deferred VFS startup, translator guard
src/common/commands.ts	+3	Use code2ProtocolConverter for command URIs
src/common/settings.ts	+22	Scheme-aware variable resolution
src/common/utilities.ts	+3	Virtual workspace getProjectRoot() fix
package.json	+2	virtualWorkspaces.supported: "limited"
.vscode-test.js	+30	Separate VFS test config
src/test/testVfsExtension/	+270	Test VFS provider + translator extension
src/test/vfs.e2e.test.ts	+150	E2E tests for VFS scenarios

Test Plan

5 E2E tests run in a pure virtual workspace (testvfs:/project):

VFS E2E tests
  ✔ Ruff should export registerUriTranslator API
  ✔ Translator should translate URIs bidirectionally
  ✔ Ruff should provide diagnostics for VFS Python file
  ✔ Ruff should format VFS Python file
  ✔ Ruff should apply auto-fix on VFS file
5 passing

Existing tests (Unit, E2E) are unaffected — no existing test files were modified.

Context

I work on the Microsoft Fabric Data Engineering VS Code extension which uses a custom VFS. We already register a URI translator with Pylance for Python IntelliSense. This PR adds the same integration pattern for Ruff.