Fixes #280859

Description

This PR addresses the lack of background customization for "Recent Sessions" in the Chat view. Previously, this section was hardcoded to use editorWidget.background. While this worked for some themes, it resulted in poor contrast or visual spacing issues in others (e.g., Light Modern or Monokai), making the section difficult to distinguish.

Changes:
I have introduced a new semantic color token: chat.agentSessionsBackground.

• Default Behavior: It falls back to editorWidget.background, ensuring no visual regressions for standard themes like Dark Modern.
• Theming Flexibility: Theme authors and users can now specifically target this area to provide better separation from the empty view if desired.

How to Test

1. Open the Chat view.
2. Verify Defaults: Switch to Dark Modern and ensure the Recent Sessions list looks identical to the current stable build.
3. Verify Customization: Open your settings.json and add the following customization:

   "workbench.colorCustomizations": {
       "chat.agentSessionsBackground": "#ff0000"
   }

4. Confirm that the Recent Sessions background updates to the custom color (red), independent of the rest of the widget.