Diff highlights entire line now instead of just the changed text

### Describe the bug

In version 0.0.420, diff highlights only changed the background color of the text that was changed. In the latest version 0.0.421, diff highlights the entire line, which makes the changes much more difficult to understand. Furthermore, the foreground color is also incorrect on the latest version, where the previous version had black-on-green (legible), the new version has white-on-green (illegible). The colors have not changed between versions, only the choice of foreground. The line numbers also appear to be aligned differently in the new version.

Version 0.0.420:

Version 0.0.421:

### Affected version

GitHub Copilot CLI 0.0.421

### Steps to reproduce the behavior

Compare output of /theme on the two versions

### Expected behavior

Original diff highlighting to be the same

### Additional context

_No response_

Start by reproducing the regression with the /theme command and comparing output from GitHub Copilot CLI 0.0.420 and 0.0.421. Trace the CLI's diff-rendering entry point to determine why whole lines, foreground colors, and line-number alignment changed; done means the newer version matches the original changed-text highlighting and remains legible.

Escrito por el modelo de indexación a partir del texto del issue.