agent: Add missing notify in ThreadHistory::delete_thread (#28144)

This would cause the history view not to get refreshed immediately when
a thread was deleted

Release Notes:

- agent: Fixed a bug where the history view wouldn't refresh after
deleting a thread

crates/agent/src/thread_store.rs

@@ -174,8 +174,9 @@ impl ThreadStore {
             let database = database_future.await.map_err(|err| anyhow!(err))?;
             database.delete_thread(id.clone()).await?;
 
-            this.update(cx, |this, _cx| {
-                this.threads.retain(|thread| thread.id != id)
+            this.update(cx, |this, cx| {
+                this.threads.retain(|thread| thread.id != id);
+                cx.notify();
             })
         })
     }