Add test coverage for experimental plugin-rsc features

[hi-ogawa]

Summary

We've landed several experimental features in @vitejs/plugin-rsc that currently lack dedicated test cases. We should add tests to exercise these code paths and ensure they work correctly.

Features needing test coverage

1. onClientReference callback (#1065)

The new setOnClientReference API allows frameworks to be notified when client reference dependencies are loaded during SSR.

import { setOnClientReference, type OnClientReference } from '@vitejs/plugin-rsc/ssr'

const callback: OnClientReference = ({ id, deps }) => {
  // id: client reference module id
  // deps: { js: string[], css: string[] }
}

setOnClientReference(callback)

Test cases needed:

• Callback is invoked when a client reference is accessed during SSR
• Callback receives correct id and deps (both js and css arrays)
• Callback is called on every access (not memoized)
• setRequireModule with onLoad callback works correctly

2. cssLinkPrecedence option (#1064)

The new cssLinkPrecedence option controls whether CSS links use React's precedence attribute.

rsc({
  cssLinkPrecedence: false, // disable React's precedence attribute on CSS links
})

Test cases needed:

• When true (default): CSS links include precedence attribute
• When false: CSS links are rendered without the precedence attribute
• Both generateResourcesCode and preloadDeps respect the setting

Related PRs

• feat(plugin-rsc): add callback to be notified when client reference dependencies are loaded #1065 - feat(plugin-rsc): add callback to be notified when client reference dependencies are loaded
• feat(plugin-rsc): allow to control whether CSS links use React's precedence attribute #1064 - feat(plugin-rsc): allow to control whether CSS links use React's precedence attribute