Agents: Session list icon spacing adjustments

[hawkticehurst]

Session list needs some tweaking:

1. Add hover background to workspace chevrons
2. Adjust search icon / filter icon to align with workspace chevron / pin icon

[mrleemurray]

Partially done!

[vs-code-engineering]

Sharing media and context from merged PR #320050 for verification:

Before:

After:

Source PR: #320050