The current WxPayCodepayRequest class only supports direct merchant mode but lacks support for service provider mode, which requires the sub_mch_id parameter. When using the codepay interface in service provider mode, users encounter the error:

受理机构必须传入sub_mch_id

This PR adds service provider mode support by:

1. Adding service provider fields to WxPayCodepayRequest:

   • sp_appid - Service provider application ID
   • sp_mchid - Service provider merchant ID
   • sub_appid - Sub-merchant application ID
   • sub_mchid - Sub-merchant ID (the key field that was missing)

2. Updating the service implementation to handle both modes:

   • Auto-detects service provider mode when any of sp_appid, sp_mchid, or sub_mchid is set
   • Uses partner API endpoint /v3/pay/partner/transactions/codepay for service provider mode
   • Uses direct API endpoint /v3/pay/transactions/codepay for direct merchant mode
   • Automatically fills missing fields from WxPayConfig

The implementation maintains backward compatibility - existing direct merchant mode usage continues to work unchanged. The new service provider mode can be used like this:

WxPayCodepayRequest request = new WxPayCodepayRequest();
request.setSpMchid("17085xxxxx");
request.setSubMchid("1230000109"); // This was the missing field causing the error
request.setDescription("订单支付");
request.setOutTradeNo("1234567890");
// ... other fields
WxPayCodepayResult result = wxPayService.codepay(request);

Fixes #3676.

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