feat(purchase): 添加关联采购合同功能

- 新增 PurchaseAssociationItem 结构体用于关联采购合同的返回结果
- 实现 Association 方法以获取关联采购合同信息
- 通过 xClient.Call 调用远程服务完成关联操作

erp/purchase.go

@@ -256,3 +256,21 @@ func (p *purchase) FactoryBatch(ctx context.Context, args ArgsPurchaseFactoryBat
 	err = xClient.Call(ctx, "FactoryBatch", args, &reply)
 	return
 }
+
+type PurchaseAssociationItem struct {
+	Id        int64  `json:"id"`
+	FactoryId int64  `json:"factoryId"`
+	SaleId    int64  `json:"saleId"`
+	PoSerial  string `json:"poSerial"`
+	BatchNo   int64  `json:"batchNo"`
+}
+
+// Association @TITLE 关联采购合同
+func (p *purchase) Association(ctx context.Context, purchaseId int64) (reply []PurchaseAssociationItem, err error) {
+	xClient, err := client.GetClient(p)
+	if err != nil {
+		return
+	}
+	err = xClient.Call(ctx, "Association", purchaseId, &reply)
+	return
+}