[APIView] Bug: Diff "Changes Only" mode incorrectly filters out added and modified lines

[AlitzelMendez]

When viewing a diff in "Changes Only" mode, only deletions are displayed while additions and modifications are incorrectly hidden.

Steps to Reproduce:

1. Navigate to: https://spa.apiviewstagingtest.com/review/8103c86200684133980931472e4f8e24?activeApiRevisionId=1d3a5565f0084512bd39a8aa9839f216&diffApiRevisionId=bdc1f25fbd7c4542897e8bb0ceb522e5&diffStyle=trees
2. Select "Full Diff" mode
3. Observe that there is a diff (addition or modification) at line 60
4. Switch to "Changes Only" mode
5. Notice that the change from line 60 disappears

Expected Behavior:

In "Changes Only" mode, all changes should be visible, including:

• Added lines (marked with +)
• Removed lines (marked with -)
• Modified lines