fix: handle non-string tool results to fix #631 (#633)

- Backend: Convert non-string content (lists, dicts) to JSON strings in _create_event_stream_message to ensure frontend always receives string content
- Frontend: Add type guard before calling startsWith() on toolCall.result for defensive programming

This fixes the TypeError: toolCall.result.startsWith is not a function when tools return complex objects.

src/server/app.py

@@ -157,6 +157,10 @@ def _create_event_stream_message(
     message_chunk, message_metadata, thread_id, agent_name
 ):
     """Create base event stream message."""
+    content = message_chunk.content
+    if not isinstance(content, str):
+        content = json.dumps(content, ensure_ascii=False)
+    
     event_stream_message = {
         "thread_id": thread_id,
         "agent": agent_name,
@@ -166,7 +170,7 @@ def _create_event_stream_message(
         "langgraph_node": message_metadata.get("langgraph_node", ""),
         "langgraph_path": message_metadata.get("langgraph_path", ""),
         "langgraph_step": message_metadata.get("langgraph_step", ""),
-        "content": message_chunk.content,
+        "content": content,
     }
 
     # Add optional fields