feat(agent): enhance event emission for final answers and fallback handling

- Updated the AgentEngine to emit final answer events even when the LLM stops without calling the final_answer tool, ensuring the frontend can render responses correctly.
- Added fallback logic in the AgentStreamHandler to emit final answers as fallback events when no answer events are streamed, improving robustness in edge cases.

internal/handler/session/agent_stream_handler.go

@@ -445,6 +445,42 @@ func (h *AgentStreamHandler) handleComplete(ctx context.Context, evt event.Event
 		}
 	}
 
+	// Fallback: if no answer events were streamed but we have a final answer,
+	// emit it as answer events so the frontend can render it properly.
+	// This guards against edge cases where the LLM stops without calling final_answer.
+	if h.finalAnswer == "" && data.FinalAnswer != "" {
+		logger.GetLogger(h.ctx).Warnf(
+			"No answer events were streamed, emitting fallback answer (len=%d)", len(data.FinalAnswer),
+		)
+		fallbackID := fmt.Sprintf("answer-fallback-%d", time.Now().UnixMilli())
+		if err := h.streamManager.AppendEvent(h.ctx, h.sessionID, h.assistantMessageID, interfaces.StreamEvent{
+			ID:        fallbackID,
+			Type:      types.ResponseTypeAnswer,
+			Content:   data.FinalAnswer,
+			Done:      false,
+			Timestamp: time.Now(),
+			Data: map[string]interface{}{
+				"event_id":    fallbackID,
+				"is_fallback": true,
+			},
+		}); err != nil {
+			logger.GetLogger(h.ctx).Errorf("Append fallback answer event failed: %v", err)
+		}
+		if err := h.streamManager.AppendEvent(h.ctx, h.sessionID, h.assistantMessageID, interfaces.StreamEvent{
+			ID:        fallbackID,
+			Type:      types.ResponseTypeAnswer,
+			Content:   "",
+			Done:      true,
+			Timestamp: time.Now(),
+			Data: map[string]interface{}{
+				"event_id":    fallbackID,
+				"is_fallback": true,
+			},
+		}); err != nil {
+			logger.GetLogger(h.ctx).Errorf("Append fallback answer done event failed: %v", err)
+		}
+	}
+
 	// Send completion event to stream manager so SSE can detect completion
 	if err := h.streamManager.AppendEvent(h.ctx, h.sessionID, h.assistantMessageID, interfaces.StreamEvent{
 		ID:        evt.ID,