assistant2: Add temporary icon for the status bar item (#22156)

This is temporary and should be removed whenever we fully transition to
Assistant 2.

<img width="800" alt="Screenshot 2024-12-17 at 16 50 48"
src="https://github.com/user-attachments/assets/61137993-ec2c-4754-9f9a-7559d1b21d89"
/>

Release Notes:

- N/A

crates/assistant2/src/assistant_panel.rs

@@ -257,7 +257,7 @@ impl Panel for AssistantPanel {
     }
 
     fn icon(&self, _cx: &WindowContext) -> Option<IconName> {
-        Some(IconName::ZedAssistant)
+        Some(IconName::ZedAssistant2)
     }
 
     fn icon_tooltip(&self, _cx: &WindowContext) -> Option<&'static str> {