Add missing sub_mchid and sub_appid fields to CombineCloseRequest.SubOrders

[Copilot]

The CombineCloseRequest.SubOrders class was missing two required fields for WeChat Pay Combined Payment Close Order API, causing 403 NOAUTH errors when calling the API.

Problem

When using the Combined Payment Close Order API (合单支付 - 关闭订单), the request was failing with a 403 error code "NOAUTH" (商户无权限) because the SubOrders class was missing the required sub_mchid and sub_appid fields.

Changes Made

Added the missing fields to CombineCloseRequest.SubOrders:

• sub_mchid (二级商户号) - Required field for service provider sub-merchant ID
• sub_appid (子商户应用ID) - Required field for sub-merchant application ID

Both fields include:

• Proper @SerializedName annotations for JSON serialization
• Complete documentation following existing code patterns
• Field validation rules matching WeChat Pay API requirements

Verification

• Manual testing confirms JSON serialization works correctly:

{
  "combine_appid": "wxd678efh567hg6787",
  "sub_orders": [{
    "mchid": "1900000109",
    "out_trade_no": "20150806125346",
    "sub_mchid": "1230000109",
    "sub_appid": "wxd678efh567hg6999"
  }]
}

• Added comprehensive test case for serialization/deserialization
• All builds pass successfully
• Zero impact on existing functionality

This fix resolves the WeChat Pay API authentication issue by providing the required merchant identification fields.

Fixes #3611.

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