refactor: 이미 완료된 건은 상태 변경 불가 로직 추가(회귀 방지)

Author: yeonjiyeon

apps/commerce-api/src/main/java/com/loopers/domain/payment/Payment.java

@@ -79,6 +79,9 @@ private void validateConstructor(Long orderId, Long userId, Money amount, String
   }
 
   public void completePayment(String pgTxnId) {
+    if (this.status == PaymentStatus.PAID || this.status == PaymentStatus.CANCELLED) {
+      return;
+    }
     this.pgTxnId = pgTxnId;
     this.status = PaymentStatus.PAID;
   }