refactor: enhance event consumption logic and add graph tool completion test

internal/ai/runtime/graphs/handoff_graph.go

@@ -79,7 +79,8 @@ func (g *HandoffGraph) Run(ctx context.Context, argumentsInJSON string) (string,
 		if err := services.ConversationService.HandoffByAI(g.conversation.ID, g.aiAgent, state.Reason); err != nil {
 			return "", err
 		}
-		return g.buildSuccessReply(), nil
+		// ConversationService sends the customer-visible handoff notice according to the dispatch decision.
+		return "", nil
 	case ConfirmationDecisionCancel:
 		return CancelHandoffReply, nil
 	default:
@@ -109,10 +110,6 @@ func (g *HandoffGraph) buildConfirmationPrompt(reason string) string {
 	return fmt.Sprintf("我准备为你转接人工客服。\n原因：%s\n请直接回复“确认”或“取消”。", strings.TrimSpace(reason))
 }
 
-func (g *HandoffGraph) buildSuccessReply() string {
-	return "已为你转接人工客服，请稍候。，请稍候。"
-}
-
 func parseHandoffDecision(value string) ConfirmationDecision {
 	return ParseConfirmationDecision(value)
 }