assistant2: Add an indicator when a response is streaming in (#23078)

This PR adds an indicator to the Assistant message to indicate that it
is still streaming:

<img width="1310" alt="Screenshot 2025-01-13 at 2 10 33 PM"
src="https://github.com/user-attachments/assets/635ee60d-b5ea-40ac-952a-b7bfa7e04fcc"
/>

Release Notes:

- N/A

crates/assistant2/src/thread.rs

@@ -132,6 +132,10 @@ impl Thread {
         self.messages.iter()
     }
 
+    pub fn is_streaming(&self) -> bool {
+        !self.pending_completions.is_empty()
+    }
+
     pub fn tools(&self) -> &Arc<ToolWorkingSet> {
         &self.tools
     }