Add functional tests for Varbase AI SEO

Problem/Motivation

Varbase AI SEO 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): the four SEO automators and the ECA model, empty SEO fields written on save, the editor's own title kept, nothing written without content, and the bulk page.
• Add a varbase-e2e-test CI job: a fresh Varbase 11 site, the recipe applied twice, then the suite. It uses the AI module test provider (EchoAI), 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.

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.