test: #3633138 Add functional tests for Varbase AI Readable Base

Issue: https://www.drupal.org/i/3633138

Adds the Varbase functional testing suite (@vardot/varbase-e2e) and a varbase-e2e-test job: a fresh Varbase 11 site, the recipe applied twice, 11 scenarios, no AI key.

• CI adds the README settings.php line for Fast 404.
• Replaces the old smoke tests, custom steps and browser-made users.

Scenarios (11):

01-01-ai-readable-base-llms-txt.feature

• Check /llms.txt is served as Markdown with the main and footer menus
• Check other .txt files are still answered by Fast 404

01-02-ai-readable-base-markdown.feature

• Check a blog post is served as Markdown at its path with .md
• Check a blog post is served as Markdown when Markdown is asked for
• Check the same path still serves HTML to browsers
• Check a taxonomy term is served as Markdown

01-03-ai-readable-base-sections.feature

• Verify a section added by the SEO admin is published in /llms.txt

01-04-ai-readable-base-access.feature

• Check the "SEO admin" user can configure /llms.txt
• Check the "Site admin" user can configure /llms.txt
• Check the site admin can change the Markdownify settings and the SEO admin cannot
• Check a content editor cannot configure /llms.txt

Green locally with gitlab-ci-local: 11 passed.

AI-Generated: Yes

Checkpoints:

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

🤖 Generated with Claude Code