fix: 恢复退款审批场景可选业务字段「资产标识」

提交 622e99b5 本意是移除与 refund_asset_type 重复的 asset_type，
但在 sceneBusinessFields 退款分支中把 asset_identifier 一并删除。

后果：保存映射时 allowedSceneBusinessField 依据该清单校验，
包含资产标识控件的映射被拒绝，报「企业微信控件映射包含当前业务不支持的字段」；
而模板中的资产标识控件为必填，不映射又会因未映射必填控件被拒，退款场景无法保存。
审批快照生成处（creation.go）仍写入 asset_identifier，故仅字段清单缺失。

本次仅在退款分支恢复该字段，asset_type 保持删除。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/application/wecom/scene.go

@@ -418,6 +418,7 @@ func sceneBusinessFields(businessType string) ([]dto.WeComBusinessFieldResponse,
 			{Code: constants.ApprovalFieldRefundNo, Name: "退款单号", ValueType: constants.ApprovalFieldValueTypeString, Description: "本次退款申请单号"},
 			{Code: constants.ApprovalFieldOrderID, Name: "订单 ID", ValueType: constants.ApprovalFieldValueTypeInteger, Description: "退款关联订单的系统 ID"},
 			{Code: constants.ApprovalFieldOrderNo, Name: "订单号", ValueType: constants.ApprovalFieldValueTypeString, Description: "退款关联订单号"},
+			{Code: constants.ApprovalFieldAssetIdentifier, Name: "资产标识", ValueType: constants.ApprovalFieldValueTypeString, Description: "退款关联卡或设备的业务标识"},
 			{Code: constants.ApprovalFieldActualReceivedAmount, Name: "订单实收金额", ValueType: constants.ApprovalFieldValueTypeMoney, Description: "以元为单位且保留两位小数的订单实收金额"},
 			{Code: constants.ApprovalFieldRequestedRefundAmount, Name: "申请退款金额", ValueType: constants.ApprovalFieldValueTypeMoney, Description: "以元为单位且保留两位小数的本次申请退款金额"},
 			{Code: constants.ApprovalFieldRefundVoucherKey, Name: "退款凭证", ValueType: constants.ApprovalFieldValueTypeFileList, Description: "提交时上传到企微文件控件的退款凭证列表"},