Desktop/VS Code sidebar: expanded session-group header row blends into its child session rows

Description:
When a session group in the sidebar (e.g. "Databricks POC") is expanded, the group's header row uses the same background/text styling as the individual session rows nested under it. There's no visual separator (background tint, border, weight change) between the header and its children, so at a glance the group header reads as just another session row, not a section boundary.

Suggested fix:
Give expanded group-header rows a distinct background color (or other treatment such as bold weight or a top border) that's visually separated from child session rows, similar to how many tree/list UIs shade parent/section rows differently from leaf rows.

Related:

• #81891 (sidebar label contrast/accessibility), different specific ask, same area of the UI.
• #66974 (closed), user-defined session groups and colors, broader ask for user-assigned colors; this issue is narrower.