test: add Next.js head change e2e coverage

[exKAZUu]

Related to suren-atoyan#272

Summary

• Added a Next.js pages-router e2e fixture that changes next/head while remounting the editor.
• Added a regression assertion that the real Monaco vs/editor/editor.main.css stylesheet remains present across repeated head changes.
• Updated the existing app-router e2e probe to load the real Monaco runtime instead of injecting a mocked Monaco object.

Why

• Issue 272 described Monaco editor styles disappearing after Next.js head updates, especially favicon changes.
• The e2e coverage now exercises the actual Monaco loader/editor path so it can catch integration regressions hidden by mocks.

Testing

• yarn check-for-ai
• yarn test:e2e:next

[gemini-code-assist]

Code Review

This pull request refactors the E2E test suite by extracting the Monaco mock into a shared fixture and introduces a new test case and page to verify that Monaco stylesheets are preserved during Next.js Head updates. Feedback was provided to optimize the stylesheet injection logic in the new test page by updating the global flag when a stylesheet is already present, thereby avoiding redundant DOM queries.

[gemini-code-assist]

To avoid redundant DOM queries on every render, the global flag should be updated if the stylesheet is already present in the document. This aligns with the goal of simplifying code and eliminating redundancy (Repository Style Guide, Rule 6).

Suggested change

	if (globalThis.__issue272StylesheetInjected) return;
	if (document.querySelector('link[data-name="vs/editor/editor.main"]')) return;
	if (globalThis.__issue272StylesheetInjected || document.querySelector('link[data-name="vs/editor/editor.main"]')) {
	globalThis.__issue272StylesheetInjected = true;
	return;
	}

References

1. Simplify code as much as possible to eliminate redundancy. ^(link)