Add isomorphic test case for vitest-hooks e2e test

Author: onlywei

e2e/react-start/vitest-hooks/src/isomorphic.ts

@@ -0,0 +1,5 @@
+import { createIsomorphicFn } from '@tanstack/react-start'
+
+export const getIsomorphicEnv = createIsomorphicFn()
+  .client(() => 'client')
+  .server(() => 'server')

e2e/react-start/vitest-hooks/tests/vitest.spec.tsx

@@ -1,15 +1,20 @@
 import { render, screen } from '@testing-library/react'
 import { describe, expect, it } from 'vitest'
+import { getIsomorphicEnv } from '../src/isomorphic'
 import { Counter } from '../src/components/Counter'
 
 /**
  * This test verifies that the tanstackStart() vite plugin works correctly
- * with Vitest. Without the fix in PR #6074, the optimizeDeps configuration
- * would cause React to be pre-bundled incorrectly, leading to "Invalid hook
- * call" errors when rendering components that use React hooks.
+ * with Vitest without forcing us to disable the plugin entirely.
+ *
+ * Without the fix in PR #6074, optimizeDeps caused React to be pre-bundled
+ * incorrectly, leading to "Invalid hook call" errors. Disabling the plugin
+ * avoids that error, but then createIsomorphicFn() is never transformed and
+ * becomes a no-op that returns undefined.
  *
  * The fix adds `process.env.VITEST !== 'true'` to disable optimizeDeps
- * in Vitest environments.
+ * in Vitest while keeping the plugin enabled, so hooks and createIsomorphicFn
+ * both work as expected.
  */
 describe('Vitest with tanstackStart() plugin', () => {
   it('renders a component using React hooks without errors', () => {
@@ -18,4 +23,8 @@ describe('Vitest with tanstackStart() plugin', () => {
     render(<Counter />)
     expect(screen.getByRole('button')).toHaveTextContent('Count: 0')
   })
+
+  it('selects the client implementation for createIsomorphicFn()', () => {
+    expect(getIsomorphicEnv()).toBe('client')
+  })
 })