This PR implements the WeChat Pay V3 subscription billing functionality (预约扣费功能/连续包月功能) as requested by the community. The subscription billing feature enables merchants to automatically charge users on a recurring basis for services like monthly subscriptions, with proper user authorization.

What's Added

Service Interface

• SubscriptionBillingService - Main service interface providing subscription billing operations

Bean Classes

Complete request/response model classes following WeChat Pay V3 API specification:

• SubscriptionScheduleRequest/Result - Schedule future billing
• SubscriptionQueryResult - Query subscription status
• SubscriptionCancelRequest/Result - Cancel subscriptions
• SubscriptionInstantBillingRequest/Result - Execute immediate billing
• SubscriptionTransactionQueryRequest/Result - Query billing history
• SubscriptionAmount - Amount and currency information
• BillingPlan - Billing plan configuration (monthly/weekly/daily/yearly)

Service Implementation

• SubscriptionBillingServiceImpl - Complete implementation following existing V3 API patterns
• Integrated into WxPayService interface and BaseWxPayServiceImpl
• Proper error handling and exception management

Features Supported

1. Schedule Subscription Billing - Create advance billing schedules for recurring payments
2. Query Subscription Status - Check subscription details and current status
3. Cancel Subscriptions - Cancel existing subscription schedules
4. Instant Billing - Execute immediate charges (useful for makeup payments)
5. Query Transaction Records - Retrieve billing history with pagination

Usage Example

// Get the subscription billing service
SubscriptionBillingService service = wxPayService.getSubscriptionBillingService();

// Create a monthly subscription
SubscriptionScheduleRequest request = new SubscriptionScheduleRequest();
request.setOutTradeNo("monthly_vip_" + System.currentTimeMillis());
request.setOpenid("user_openid");
request.setDescription("VIP会员 - 连续包月");
request.setScheduleTime("2024-09-01T10:00:00+08:00");

// Set amount
SubscriptionAmount amount = new SubscriptionAmount();
amount.setTotal(3000); // 30 RMB in cents
amount.setCurrency("CNY");
request.setAmount(amount);

// Configure monthly billing plan
BillingPlan plan = new BillingPlan();
plan.setPlanType("MONTHLY");
plan.setPeriod(1);
plan.setTotalCount(12); // 12 months
request.setBillingPlan(plan);

// Execute the subscription
SubscriptionScheduleResult result = service.scheduleSubscription(request);

Implementation Details

The implementation follows WeChat Pay V3 API documentation: https://pay.weixin.qq.com/doc/v3/merchant/4012161105

• API Endpoints: All 5 core subscription billing endpoints are supported
• Authentication: Uses existing V3 signature and certificate handling
• Error Handling: Leverages existing WxPayException framework
• Code Style: Passes all checkstyle validations and follows project patterns
• Testing: Includes basic test structure (disabled by default due to credential requirements)
• Documentation: Comprehensive usage guide with examples provided

Code Quality

✅ Builds successfully with existing codebase
✅ Passes all checkstyle validations
✅ Follows established architectural patterns
✅ Comprehensive JavaDoc documentation
✅ Proper exception handling

This implementation provides production-ready support for WeChat's subscription billing capabilities while maintaining full compatibility with the existing WxJava architecture.

Fixes #3593.

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