fix: 修复订阅支付商品名前后缀不生效

[wucm667]

背景

修复 issue Wei-Shaw#2435。后台已有支付商品名前缀和后缀配置，但订阅套餐支付生成商品名时会直接返回套餐 ProductName 或默认套餐名，没有套用 PRODUCT_NAME_PREFIX / PRODUCT_NAME_SUFFIX。

修改内容

• 订阅套餐支付商品名统一经过支付商品名前后缀拼接逻辑
• 保持余额充值商品名的既有展示行为
• 增加订阅套餐自定义商品名和默认商品名的单元测试覆盖

测试

• go test ./internal/service -run 'TestBuildPaymentSubject|TestCalculateCreateOrderPayAmount|TestBuildCreateOrderResponse'
• go test ./internal/service

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[wucm667]

已改为提交到 upstream 仓库：Wei-Shaw#2449