CI: Add verification CI job

[VeckoTheGecko]

Description

xref #2633 (comment)

Checklist

• xref Add "validation" pytest marker #2633 (comment)
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
  • Describe how you used it (e.g., by pasting your prompt): Asked claude to create an additional workflow mimicking the flaky one and using the Pixi task.

[VeckoTheGecko]

Green ticks on CI - keen to know what you think @fluidnumericsJoe and @erikvansebille

This will become a "required check" in the GitHub CI

[erikvansebille]

Looks good to me!

[fluidnumericsJoe]

This looks good. I like the idea of separating out the validation tests from the other types of tests. Thinking about future triage, seeing failure in validation tests among other pass/fail items is a useful symptom that helps assist future developers in determining where an issue might be.