fix(fast-html): add errors fixture for f-template error cases

[janechu]

Pull Request

📖 Description

Adds an errors test fixture for @microsoft/fast-html that covers the two error conditions thrown by the <f-template> element:

• No template element present — when an <f-template> contains no <template> child, a noTemplateProvided error (code 2000) is thrown.
• Multiple template elements present — when an <f-template> contains more than one <template> child, a moreThanOneTemplateProvided error (code 2001) is thrown.

The fixture includes index.html, main.ts, and errors.spec.ts. Tests use page.addInitScript to intercept unhandledrejection events (the errors originate inside an unhandled promise chain) for reliable cross-browser coverage across Chromium, Firefox, and WebKit.

📑 Test Plan

Run the new fixture tests:

cd packages/fast-html
npx playwright test test/fixtures/errors/errors.spec.ts

All 6 tests pass (2 cases × 3 browsers).

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.