fix test failures caused by monaco editor ESM CSS imports

Resolves GH-16401

- Add ESM loader hooks in @theia/test-setup to handle .css imports
  from @theia/monaco-editor-core ESM bundles during mocha test runs
- Without this, Node's ESM resolver fails on .css imports, causing
  mocha's import→require fallback to partially execute test files
  twice, leading to duplicate side effects like
  FrontendApplicationConfigProvider.set() being called twice

Contributed on behalf of STMicroelectronics

Author: ndoschek

dev-packages/private-test-setup/esm-loader-hooks.mjs

@@ -0,0 +1,34 @@
+// *****************************************************************************
+// Copyright (C) 2026 STMicroelectronics and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// http://www.eclipse.org/legal/epl-2.0.
+//
+// This Source Code may also be made available under the following Secondary
+// Licenses when the conditions for such availability set forth in the Eclipse
+// Public License v. 2.0 are satisfied: GNU General Public License, version 2
+// with the GNU Classpath Exception which is available at
+// https://www.gnu.org/software/classpath/license.html.
+//
+// SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-only WITH Classpath-exception-2.0
+// *****************************************************************************
+
+// ESM loader hooks to handle non-JS file extensions (e.g. .css) that
+// are imported by ESM dependencies such as @theia/monaco-editor-core.
+// Without this, Node's ESM resolver throws ERR_UNKNOWN_FILE_EXTENSION
+// which causes mocha's import-then-require fallback to partially execute
+// test files before retrying, leading to duplicate side effects.
+
+const STYLE_EXTENSIONS = ['.css', '.scss', '.sass', '.less'];
+
+export function resolve(specifier, context, nextResolve) {
+    return nextResolve(specifier, context);
+}
+
+export function load(url, context, nextLoad) {
+    if (STYLE_EXTENSIONS.some(ext => url.endsWith(ext))) {
+        return { format: 'module', source: 'export default {};', shortCircuit: true };
+    }
+    return nextLoad(url, context);
+}

dev-packages/private-test-setup/test-setup.js

@@ -14,5 +14,13 @@
 // SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-only WITH Classpath-exception-2.0
 // *****************************************************************************
 
+// Register ESM loader hooks so that non-JS imports (e.g. .css files from
+// @theia/monaco-editor-core ESM bundles) are handled before mocha attempts
+// to load test files. Without this, Node's ESM resolver fails on .css
+// imports, and mocha's import→require fallback causes files to be partially
+// executed twice, leading to side-effect duplication.
+const { register } = require('node:module');
+register('./esm-loader-hooks.mjs', require('node:url').pathToFileURL(__filename));
+
 // Mock DragEvent as '@lumino/dragdrop' already requires it at require time
 global.DragEvent = class DragEvent { };