This PR adds comprehensive support for overseas WeChat Pay (境外微信支付) to address the limitations mentioned in the issue where new overseas merchant accounts can only use the new global API endpoints.

Problem

The existing createOrderV3 method only supports domestic WeChat Pay endpoints and is missing required parameters for overseas payments:

• Missing trade_type parameter
• Missing merchant_category_code parameter
• Uses domestic endpoints (/v3/pay/transactions/*) instead of global endpoints
• Cannot specify overseas base URL (https://apihk.mch.weixin.qq.com)

Solution

Added new classes and methods specifically for overseas WeChat Pay while maintaining full backward compatibility:

New Classes

1. GlobalTradeTypeEnum - Enum for overseas payment types with global endpoints:

   APP("/global/v3/transactions/app")
   JSAPI("/global/v3/transactions/jsapi") 
   NATIVE("/global/v3/transactions/native")
   H5("/global/v3/transactions/h5")

2. WxPayUnifiedOrderV3GlobalRequest - Extended request class with overseas-specific fields:

   @SerializedName(value = "trade_type")
   private String tradeType;
   
   @SerializedName(value = "merchant_category_code") 
   private String merchantCategoryCode;

New API Methods

• createOrderV3Global(GlobalTradeTypeEnum, WxPayUnifiedOrderV3GlobalRequest) - High-level method for overseas orders
• unifiedOrderV3Global(GlobalTradeTypeEnum, WxPayUnifiedOrderV3GlobalRequest) - Low-level API call using overseas base URL

Usage Example

// Create overseas payment request
WxPayUnifiedOrderV3GlobalRequest request = new WxPayUnifiedOrderV3GlobalRequest();
request.setOutTradeNo(RandomUtils.getRandomStr());
request.setDescription("境外商品购买");

// Set required overseas parameters
request.setTradeType("JSAPI");
request.setMerchantCategoryCode("5812"); // Required for overseas merchants

// Set amount and payer
WxPayUnifiedOrderV3GlobalRequest.Amount amount = new WxPayUnifiedOrderV3GlobalRequest.Amount();
amount.setTotal(100);
request.setAmount(amount);

WxPayUnifiedOrderV3GlobalRequest.Payer payer = new WxPayUnifiedOrderV3GlobalRequest.Payer();
payer.setOpenid("user_openid");
request.setPayer(payer);

// Call overseas payment API
WxPayUnifiedOrderV3Result.JsapiResult result = payService.createOrderV3Global(
    GlobalTradeTypeEnum.JSAPI, request
);

Key Features

• ✅ Backward Compatible - No changes to existing domestic payment functionality
• ✅ Complete Coverage - Supports all payment types (JSAPI, APP, NATIVE, H5)
• ✅ Automatic URL Handling - Uses https://apihk.mch.weixin.qq.com for overseas payments
• ✅ Proper Parameter Support - Includes trade_type and merchant_category_code
• ✅ Consistent API Design - Follows existing patterns and conventions
• ✅ Comprehensive Examples - Includes usage examples and documentation

Testing

• Added unit tests verifying new request class functionality
• Created comprehensive usage examples for all payment types
• Verified compilation and backward compatibility

Fixes #3618.

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