Add functional tests for Varbase AI Readable Base

Problem/Motivation

Varbase AI Readable Base has a few smoke tests in tests/features and no CI job runs them. Nothing checks that the recipe applies on a fresh Varbase 11 site, twice, and that editors, admins and visitors get what it promises.

Proposed resolution

• Add the Varbase functional testing suite (Playwright and Cucumber through @vardot/varbase-e2e): /llms.txt, Markdown at .md and with Accept: text/markdown, an SEO admin adding a section, and who configures it.
• Add a varbase-e2e-test CI job: a fresh Varbase 11 site, the recipe applied twice, then the suite. It adds the README settings.php line for Fast 404, so no AI key is needed.
• Scenarios that need a real AI provider are tagged @ai-provider and run in their own job only when an OPENAI_API_KEY CI variable is set.

Found while testing:

• The Markdown of a blog post starts with the breadcrumb and carries the lazy-load SVG placeholder as a data URI instead of the image.

Remaining tasks

• ✅ File an issue
• ❌ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ❌ Automated unit/functional testing coverage
• ❌ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Added the Varbase functional testing suite and a CI job that runs it on a fresh Varbase 11 site.