[Tests] Cover theme dev error page rendering and HTML escaping

[github-actions]

WHY are these changes introduced?

getErrorPage renders the HTML page shown when theme dev fails to render a page, and it had no test coverage. Its escapeHtml helper is the only thing preventing untrusted error text (Liquid error messages, stack traces, file paths from the theme) from being injected as markup into that page, and nothing guarded that behavior against regression.

WHAT is this pull request doing?

Adds error-page.test.ts next to error-page.ts, covering the public contract of getErrorPage:

• The title and header appear in the rendered page.
• Every error's message and code is rendered, not just the first.
• HTML in error messages and codes is escaped, so a <script> tag in an error cannot inject markup.
• A complete document is still returned when the error list is empty.

Removing the escapeHtml calls from error-page.ts makes the escaping test fail, so the test earns its keep. No production code is changed.

How to manually test your changes?

CI

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

[Suleimanlatrsh]

Tested locally, including HTML in the error messages and code. Looks good 👍