Fix panic in context server configuration (#34118)

Release Notes:

- Fixed a panic that could occur when configuring MCP servers

crates/agent_ui/src/agent_configuration/configure_context_server_modal.rs

@@ -740,7 +740,9 @@ fn wait_for_context_server(
     });
 
     cx.spawn(async move |_cx| {
-        let result = rx.await.unwrap();
+        let result = rx
+            .await
+            .map_err(|_| Arc::from("Context server store was dropped"))?;
         drop(subscription);
         result
     })