feat(agent): Unselect session when archiving currently selected session

When a session is archived and it was the currently selected session,
it is now automatically deselected and the empty state placeholder is
shown in the right panel.

- Updated SessionManagerProps to allow null for onSelectSession callback
- Added logic to deselect session on archive if it was selected
- Added data-testid attributes for testing

app/src/components/views/agent-view.tsx

@@ -471,7 +471,7 @@ export function AgentView() {
 
         {/* Messages */}
         {!currentSessionId ? (
-          <div className="flex-1 flex items-center justify-center">
+          <div className="flex-1 flex items-center justify-center" data-testid="no-session-placeholder">
             <div className="text-center">
               <Bot className="w-12 h-12 text-muted-foreground mx-auto mb-4 opacity-50" />
               <h2 className="text-lg font-semibold mb-2">