feat(agent): remove redundant done message from stream_chat

The `stream_chat` method was yielding a "done" message after streaming all chunks, but this is unnecessary as the streaming completion is already indicated by the end of the stream. Removing this redundant message simplifies the response handling and aligns with typical streaming patterns.

gns3server/agent/gns3_copilot/agent_service.py

@@ -160,8 +160,6 @@ class AgentService:
                 if chunk:
                     yield chunk
 
-            yield {"type": "done", "session_id": session_id}
-
         except Exception as e:
             log.error("Error in stream_chat: %s", e, exc_info=True)
             yield {"type": "error", "error": str(e), "session_id": session_id}