Increase diff limits for Rapid Diffs

Summary

Based on the performance analysis in the spike issue #534953 (closed), we've determined that Rapid Diffs frontend can handle significantly more files and lines than current limits allow. The current bottleneck appears to be on the backend rather than frontend rendering.

Goal

Increase the diff limits for Rapid Diffs to improve the user experience while maintaining acceptable performance.

Performance testing showed:

1. Rapid Diffs significantly outperform legacy diffs at all file sizes
2. Frontend remains responsive with expanded diffs up to ~600 files × 400 lines before GDK issues occurred

Risks

1. Increased server-side resource usage for large diffs
2. Impact on Gitaly performance with larger diff sizes?

Related Issues and MRs

• Spike: Refine diff limits for Rapid Diffs
• Related MR: Use content-visibility on Rapid Diffs files