Fix broken QA selector

What does this MR do and why?

Recent UI changes moved a QA selector in app/assets/javascripts/ci/pipeline_editor/components/header/validation_segment.vue

After the change, if the validation content was "Pipeline syntax is correct. Learn more", the selector found Learn more instead of Pipeline syntax is correct.

This moves the selector back to an outer element so that it finds the whole text (message and link).

Fixes #393460 (closed)

How to set up and validate locally

In the qa/ directory:

bundle exec rspec qa/specs/features/browser_ui/4_verify/pipeline/pipeline_editor_tabs_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.