Add border to diff expansion rows

What does this MR do and why?

Adds a border to the diff expansion rows to show the cut off between these rows and the diff content.

Screenshots or screen recordings

How to set up and validate locally

• View a merge request that has a expansion button

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.

Closes #371704 (closed)