错误处理

{
  "code": 40001,
  "message": "Validation failed",
  "error": {
    "field": "productId",
    "reason": "Product ID is required"
  }
}

{
  "code": 40001,
  "message": "Bad Request",
  "error": {
    "field": "price",
    "reason": "Price must be at least 0.01"
  }
}

{
  "code": 40401,
  "message": "Resource not found"
}

{
  "code": 42901,
  "message": "Rate limit exceeded. Try again later."
}

{
  "code": 0,
  "message": "success",
  "data": {
    // 响应数据
  }
}

{
  "code": 0,
  "message": "success",
  "data": {
    "items": [...],
    "pagination": {
      "page": 1,
      "size": 20,
      "total": 42,
      "totalPages": 3
    }
  }
}