Add descriptive labels to line numbers in Rapid Diffs

Stanislav Lashmanovrequested to merge
slashmanov/rapid-diffs-descriptive-line-numbers into master

What does this MR do and why?

This MR adds more descriptive labels to line numbers in Rapid Diffs.

Fixes: #466593 (closed)

Screenshots or screen recordings

Before After
Screen_Recording_2025-05-07_at_18.05.39 Screen_Recording_2025-05-07_at_18.00.47

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. Navigate through diff file using screen reader
  6. Observe that diff line numbers are properly announced

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