assistant2: Add text_ellipsis to the PastThread label (#22117)

To treat the case for whenever the assistant panel gets small.

<img width="360" alt="Screenshot 2024-12-16 at 20 12 34"
src="https://github.com/user-attachments/assets/5426a5b9-9baf-41e3-a2c6-2f997378c994"
/>

Release Notes:

- N/A

crates/assistant2/src/thread_history.rs

@@ -126,7 +126,7 @@ impl RenderOnce for PastThread {
                     .color(Color::Muted),
             )
             .spacing(ListItemSpacing::Sparse)
-            .child(Label::new(summary).size(LabelSize::Small))
+            .child(Label::new(summary).size(LabelSize::Small).text_ellipsis())
             .end_slot(
                 h_flex()
                     .gap_2()