Add some docs to better understand this pattern

Author: bendvc

packages/mrt-utilities/test/helpers/express4-alias.cjs

@@ -1,7 +1,22 @@
+/*
+ * Copyright (c) 2025, Salesforce, Inc.
+ * SPDX-License-Identifier: Apache-2
+ * For full license text, see the license.txt file in the repo root or http://www.apache.org/licenses/LICENSE-2.0
+ */
+
 const Module = require('module');
 
+/**
+ * Monkey-patch Node's module resolution for tests.
+ * - Redirect `express` to the `express4` alias.
+ * - Preserve subpath imports like `express/static` by mapping to `express4/...`.
+ */
 const originalResolveFilename = Module._resolveFilename;
 
+/**
+ * Ensure Mocha runs the same test suite against Express 4 by mapping the
+ * module IDs at resolution time. This avoids touching production imports.
+ */
 Module._resolveFilename = function (request, parent, isMain, options) {
   if (request === 'express') {
     return originalResolveFilename.call(this, 'express4', parent, isMain, options);