Add text to loading state for pipeline mini graph

What does this MR do and why?

Add's text to loading state for pipeline mini graph. Also adds missing test coverage.

Screenshots or screen recordings

How to set up and validate locally

Have a project with CI and jobs. Visit CI/CD > Pipelines, then click on a stage.

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 #363664 (closed)