fix: update unit tests for WidgetPlugin changes

- Add moduleNameMapper in jest.config.cjs to resolve React 18 from the
  plugin's local node_modules, fixing dual-React-instance errors
- Mock usePersistentState in @deephaven/plugin mock to avoid FiberProvider
  dependency in tests
- Update DocumentHandler test to reflect new behavior where non-layout
  children are rendered directly without ReactPanel wrapping

Author: mofojed

plugins/ui/src/js/__mocks__/@deephaven/plugin.js

@@ -1,8 +1,12 @@
 // Mock @deephaven/plugin package
+const React = require('react');
 const PluginActual = jest.requireActual('@deephaven/plugin');
 
 module.exports = {
   ...PluginActual,
   useDashboardPlugins: jest.fn(() => []),
+  // Mock usePersistentState to behave like useState.
+  // The real implementation requires FiberProvider which is internal to Dashboard.
+  usePersistentState: (initialState, _config) => React.useState(initialState),
   __esModule: true,
 };

plugins/ui/src/js/jest.config.cjs

@@ -1,7 +1,17 @@
+const path = require('path');
 const baseConfig = require('../../../../jest.config.base.cjs');
 const packageJson = require('./package');
 
 module.exports = {
   ...baseConfig,
   displayName: packageJson.name,
+  moduleNameMapper: {
+    ...baseConfig.moduleNameMapper,
+    // Force all react imports to the local React 18 to avoid dual-instance issues
+    // with the root workspace's React 17.
+    '^react$': path.resolve(__dirname, 'node_modules/react'),
+    '^react/(.*)$': path.resolve(__dirname, 'node_modules/react/$1'),
+    '^react-dom$': path.resolve(__dirname, 'node_modules/react-dom'),
+    '^react-dom/(.*)$': path.resolve(__dirname, 'node_modules/react-dom/$1'),
+  },
 };

plugins/ui/src/js/src/widget/DocumentHandler.test.tsx

@@ -72,10 +72,12 @@ it('should throw an error if the document mixes panel and non-panel elements', (
   );
 });
 
-it('should combine multiple single elements into one panel', () => {
+it('should render multiple single elements directly without wrapping in a panel', () => {
   const children = makeDocument([makeElement('foo'), makeElement('bar')]);
   render(makeDocumentHandler({ children }));
-  expect(mockReactPanel).toHaveBeenCalledTimes(1);
+  // When not nested (no PanelIdContext), non-layout children are rendered directly
+  // without being wrapped in a ReactPanel (WidgetPlugin handles the panel)
+  expect(mockReactPanel).toHaveBeenCalledTimes(0);
 });
 
 it('should render multiple panels', () => {