[test] Update e2e test app

[Ad1tya-007]

Fix: Use ReactDOMClient.createRoot instead of outdated API

Summary

Removes webpack warnings by replacing an outdated React API check with the standard ReactDOMClient.createRoot API.

Changes

• Removed check for ReactDOM.unstable_createRoot (doesn't exist in react-dom)
• Use ReactDOMClient.createRoot directly (standard API in React 18+)
• Removed unused ReactDOM import

Problem

Webpack was warning:

export 'unstable_createRoot' (imported as 'ReactDOM') was not found in 'react-dom'

The code was checking for ReactDOM.unstable_createRoot, which doesn't exist in the current version of react-dom.

Solution

Since we're using React 19, we can use ReactDOMClient.createRoot directly, which is the standard API for React 18+.

Testing

• E2e tests still pass
• Webpack warnings about unstable_createRoot are eliminated
• Cleaner build output

---

This PR now only focuses on fixing the React DOM client API usage.

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

Netlify deploy preview

https://deploy-preview-47252--material-ui.netlify.app/

Bundle size report

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

Generated by 🚫 dangerJS against 76f0175