Support loading collapsed files in Rapid Diffs

What does this MR do and why?

This MR implements #507928 (closed).

It allows to load collapsed files in Rapid Diffs.

Screenshots or screen recordings

Screen_Recording_2025-05-22_at_00.34.03

How to set up and validate locally

1. Enable rapid_diffs and rapid_diffs_on_mr_show feature flags
2. Go to a large merge request (at least 100 files changes, more than 1000 lines changed)
3. Select 'Changes' tab
4. Add ?rapid_diffs=true to the URL, follow it
5. Scroll to any collapsed file
6. Click 'Show changes' button
7. Observe expanded file