Improve border radius for stuck header in Rapid Diffs

What does this MR do and why?

This MR fixes stuck file header in Rapid Diffs using container queries:

1. Bottom border radius matches the border bottom radius of the whole file
2. Top border radius is not rounded for better visuals

Screenshots or screen recordings

Before	After
Screen_Recording_2025-05-19_at_15.28.22	Screen_Recording_2025-05-19_at_15.25.03

How to set up and validate locally

1. Enable rapid_diffs and rapid_diffs_on_mr_show feature flags
2. Go to any merge request
3. Select 'Changes' tab
4. Add ?rapid_diffs=true to the URL, follow it
5. Scroll through files
6. Observe correct border radius for diff file header when it's stuck