fix(vite-plugin-react-router): preserve existing rollup input entries during SSR build

[nahyeongjin1]

Description

The plugin was overwriting the entire rollup.input config during SSR build, which removed entries set by other plugins.
This caused react-router v7's prerender feature to fail because the server-build entry was being removed.

This PR fixes the issue by preserving existing rollup input entries and merging them with the Netlify function handler entry.
The fix handles all possible input formats:

• String input
• Array input
• Object input

Additionally, this PR enables the previously skipped prerender e2e tests for both serverless and edge sites, which were disabled due to this bug (referenced in React Router issue #13226).

Related Tickets & Documents

• Related Issue [BUG] rollupOptions.input overwrite breaks React Router prerender #606
• Closes [BUG] rollupOptions.input overwrite breaks React Router prerender #606

QA Instructions, Screenshots, Recordings

Reproduction Steps

1. Create a new project using the official Netlify template:

npx create-react-router@latest --template netlify/react-router-template

2. Navigate to the project folder (default: my-react-router-app)

3. Replace react-router.config.ts with:

import type { Config } from "@react-router/dev/config";

export default {
  ssr: true,
  future: {
    v8_middleware: true,
  },
  async prerender() {
    return ["/"];
  },
} satisfies Config;

4. Run the build:

npm install && npm run typecheck && npm run build

Before this fix

Build fails with:

✗ Build failed in 283ms
[react-router] Server build file not found in manifest

After this fix

Build succeeds with prerender working:

build/server/server.js                         0.35 kB
build/server/assets/server-build-lv_U4l46.js  14.57 kB   ← preserved

Prerender (html): / -> build/client/index.html
✓ built in 292ms

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[nahyeongjin1]

Hi! I've added e2e tests that enable the previously skipped prerender tests.
Could someone take a look when you have time? Happy to make any changes if needed.