assistant2: Add prompt editor history (#23439)

This PR adds the prompt editor history to Assistant2.

<img width="1309" alt="Screenshot 2025-01-21 at 9 02 07 PM"
src="https://github.com/user-attachments/assets/d79936fe-1c23-425f-b99d-43f85afd0c39"
/>

Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -1342,7 +1342,6 @@ impl AssistantPanelDelegate for ConcreteAssistantPanelDelegate {
             return;
         };
 
-        // Activate the panel
         if !panel.focus_handle(cx).contains_focused(cx) {
             workspace.toggle_panel_focus::<AssistantPanel>(cx);
         }