Full-line red/green backgrounds for diff reviewer (or make configurable) in Codex VS Code extension

Summary

The current diff reviewer in the Codex VS Code extension only highlights the line numbers/gutter with red or green for changes, rather than using full-line red/green background fills. This makes it noticeably harder to scan diffs compared to the traditional stronger background style that most diff tools and previous versions used.

Screenshot for reference:

<img alt="Codex reviewer diff highlighting, only gutter colored" width="1091" src="https://github.com/user-attachments/assets/733a7522-fdbc-4bb7-985b-51f6f71998b1" />

Request

• Option 1: Restore the previous behavior where the entire line for insertions and deletions is highlighted in green/red, not just the line-number gutter.
• Option 2: Make this behavior a configuration option, so users can choose between full line backgrounds and the current subtle accent-on-numbers style.

The current approach is much less readable for large diffs or for users with color-accommodation needs.

Why this matters

Full-line highlights make it significantly easier to spot changes, especially in large or visually dense diffs. Many users are accustomed to, and expect, the more strongly-highlighted red/green lines as seen in traditional diff tools and GitHub's old PR UI.

If performance or accessibility triggered this change, please consider at least making the behavior configurable.

Thanks for considering this request!