Validate ci_builds partitioning constraint

What does this MR do and why?

This is a follow-up from !114077 (merged) in which we added the partitioning constraint and scheduled its async validation. I was validated during the weekend and now we're ready to merge its synchronously validation for self-managed and everyone else.

This will be a no-op on .com since it's already valid.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

Related to #382033 (closed)