fix: fs import for mock-fs

`mock-fs` uses `process.binding('fs')`, so it cannot support mocking the `node:fs` import alias it seems. this should fix test failures in CI (for node 22 at least)

Author: acao

packages/graphql-language-service-server/src/MessageProcessor.ts

@@ -6,8 +6,8 @@
  *  LICENSE file in the root directory of this source tree.
  *
  */
-
-import { existsSync, mkdirSync } from 'node:fs';
+// do not change to node:fs import, or it will break mock-fs
+import { existsSync, mkdirSync } from 'fs';
 import { mkdir, readFile, writeFile } from 'node:fs/promises';
 import * as path from 'node:path';
 import { URI } from 'vscode-uri';