test: add new NotificationService instantiation test

backend/internal/services/notification_service_test.go

@@ -117,3 +117,11 @@ func TestNotificationService_MarkAllAsRead(t *testing.T) {
 		}
 	})
 }
+
+// NewNotificationService instantiation test
+func TestNewNotificationService(t *testing.T) {
+	service := NewNotificationService(nil, nil)
+	if service == nil {
+		t.Error("Expected NotificationService to not be nil")
+	}
+}