feat(agent): handle AIMessageChunk content access in copilot streaming

Update agent service to properly access content from AIMessageChunk objects during chat model streaming. Instead of using dictionary get method on the chunk, now use getattr to directly access the content attribute, ensuring compatibility with the AIMessageChunk object structure.

gns3server/agent/gns3_copilot/agent_service.py

@@ -171,7 +171,8 @@ class AgentService:
         if event_type == "on_chat_model_stream":
             # Streaming text content from LLM
             chunk = data.get("chunk", {})
-            content = chunk.get("content", "")
+            # chunk is AIMessageChunk object, access content directly
+            content = getattr(chunk, "content", "")
             if content:
                 return {"type": "content", "content": content}