Update chatColors.ts regarding Issue #251146

[Neighbor07]

This PR updates the chatColors.ts file to adjust the color definitions for Chat UI elements. These changes help improve consistency with the rest of the workbench theme and improve readability in both light and dark themes.

• Updated color values in chatColors.ts.
• Aligned Chat UI color tokens with existing workbench theme guidelines.
• Improved contrast and accessibility where appropriate.
  The code is up-to-date with the main branch.
  Steps to test the proposed changes

1. Build and run VS Code from source.
2. Open a session in the workbench.
3. Verify that updated colors display correctly in both:
   • Light theme
   • Dark theme
     Updated chatColors.ts: to include chat.responseBackground, chat.promptForeground, chat.responseForeground, and chat.responseSecondaryForeground.

[Neighbor07]

@microsoft-github-policy-service agree