fix(ticket): close backend core review gaps

internal/services/ticket_service.go

@@ -462,7 +462,7 @@ func (s *ticketService) assignTicketTx(tx *gorm.DB, req request.AssignTicketRequ
 	if ticket == nil {
 		return nil, errorsx.InvalidParam("工单不存在")
 	}
-	if err := s.validateAssignee(req.ToUserID); err != nil {
+	if err := s.validateRequiredAssignee(req.ToUserID); err != nil {
 		return nil, err
 	}
 	now := time.Now()
@@ -586,6 +586,13 @@ func (s *ticketService) validateAssignee(userID int64) error {
 	if userID <= 0 {
 		return nil
 	}
+	return s.validateRequiredAssignee(userID)
+}
+
+func (s *ticketService) validateRequiredAssignee(userID int64) error {
+	if userID <= 0 {
+		return errorsx.InvalidParam("负责人不存在")
+	}
 	user := UserService.Get(userID)
 	if user == nil || user.Status == enums.StatusDeleted {
 		return errorsx.InvalidParam("负责人不存在")

internal/services/ticket_service_test.go

@@ -190,6 +190,24 @@ func TestTicketServiceAddProgressStoresContentAndAuthor(t *testing.T) {
 	}
 }
 
+func TestTicketServiceAssignTicketRequiresTargetUser(t *testing.T) {
+	setupTicketTestDB(t)
+	operator := createTestOperator(t, "assign-operator")
+	ticket, err := services.TicketService.CreateTicket(createTestTicketRequest("assign-ticket"), operator)
+	if err != nil {
+		t.Fatalf("CreateTicket() error = %v", err)
+	}
+
+	err = services.TicketService.AssignTicket(request.AssignTicketRequest{
+		TicketID: ticket.ID,
+		ToUserID: 0,
+		Reason:   "invalid assignment",
+	}, operator)
+	if err == nil {
+		t.Fatalf("expected AssignTicket() to reject empty target user")
+	}
+}
+
 func TestTicketServiceSummaryCountsStaleTickets(t *testing.T) {
 	setupTicketTestDB(t)
 	operator := createTestOperator(t, "summary-operator")