Agents: Indent chat session menu items relative to workspace labels

[hawkticehurst]

To improve visual hierarchy / scannability of chat session list items, indent chat sessions so there is a clearer parent child relationship between sessions and their workspace.

[vs-code-engineering]

Sharing media and context from merged PR #320006 for verification:

Adds another 6px of left padding to chat session list items to improve hierarchy/scannability and create a clearer parent-child relationship between sessions and workspaces.

Source PR: #320006