Handle empty prompt in chat streaming with error callback

- Add error handling for empty prompts in streamChat method
- Ensure proper error message generation and callback invocation
- Improve error propagation in chat streaming scenarios

neo/assistant/api.go

@@ -631,6 +631,7 @@ func (ast *Assistant) streamChat(
 
 		// if the prompt is empty, return the error
 		if prompt == "" {
+			chatMessage.New().Error(retry.Error()).Done().Callback(cb).Write(c.Writer)
 			return nil, retry
 		}