test(rsc): test React.cache

[Copilot]

Hiding Copilot summary since it's bad. Tests are rewritten by Hiroshi

Adds comprehensive test coverage for React.cache functionality in the React Server Components plugin. This ensures that the React 19 caching API works correctly in the RSC environment.

What's Added

New Test Route

• packages/plugin-rsc/examples/basic/src/routes/react-cache/server.tsx:
  • TestReactCacheFn: Interactive form that tests React.cache() with different arguments and server actions
  • TestReactCacheComponent: Demonstrates React.cache working within server component render cycles
  • Shows caching behavior with async functions and multiple calls

E2E Test Coverage

• Updated packages/plugin-rsc/e2e/basic.test.ts:
  • Tests React.cache function interactivity via form submissions
  • Tests React.cache in server component rendering context
  • Validates functionality works in both dev and build modes
  • All tests passing (4/4)

Integration

• Updated packages/plugin-rsc/examples/basic/src/routes/root.tsx to include the new test component

Example Usage

The implementation demonstrates React.cache working correctly:

const testCacheFn = React.cache(async (arg: unknown) => {
  cacheFnCount++
  await new Promise((resolve) => setTimeout(resolve, 20))
  return arg
})

// Multiple calls with same argument use cache
await testCacheFn('same-arg')
await testCacheFn('same-arg') // Cached result

This provides confidence that developers can use React.cache() in their RSC applications with @vitejs/plugin-rsc without issues.

Fixes #619.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.