修复(#138): 补齐 errMsg 漏掉的错误码映射

为避免业务错误被映射成 "Internal Server Error"，补齐
pkg/xerr/errMsg.go 中漏写的 5 个 errCode：

- UserCommissionNotEnough (20010): 佣金余额不足
- SendSmsError            (90002): 短信发送失败
- AreaCodeIsEmpty         (90009): 国家区号不能为空
- DeviceBindLimitExceeded (90019): 设备绑定数量已达上限
- ExistAvailableTraffic   (61005): 存在可用流量

不改动既有条目；不动 errCode.go/errors.go；不改 MapErrMsg 的 fallback 策略。
新增单测覆盖这 5 个码并验证未定义码仍安全 fallback。

Co-authored-by: multica-agent <github@multica.ai>

pkg/xerr/errMsg.go

@@ -37,6 +37,7 @@ func init() {
 		TelegramNotBound:              "Telegram not bound ",
 		UserNotBindOauth:              "User not bind oauth method",
 		InviteCodeError:               "Invite code error",
+		UserCommissionNotEnough:       "佣金余额不足",
 		RegisterIPLimit:               "Too many registrations",
 		EmailBindError:                "Email already bound",
 		UserBindInviteCodeExist:       "Invite code already bound",
@@ -82,6 +83,8 @@ func init() {
 		// System error
 		DebugModeError: "Debug mode is enabled",
 
+		SendSmsError: "短信发送失败",
+
 		GetAuthenticatorError:          "Unsupported login method",
 		AuthenticatorNotSupportedError: "The authenticator does not support this method",
 
@@ -94,15 +97,18 @@ func init() {
 		TelephoneExist:                     "Telephone already exists",
 		DeviceExist:                        "device exists",
 		PasswordIsEmpty:                    "password is empty",
+		AreaCodeIsEmpty:                    "国家区号不能为空",
 		TelephoneError:                     "telephone number error",
 		DeviceNotExist:                     "Device does not exist",
 		UseridNotMatch:                     "Userid not match",
+		DeviceBindLimitExceeded:            "设备绑定数量已达上限",
 
 		// Order error
 		OrderNotExist:                 "Order does not exist",
 		PaymentMethodNotFound:         "Payment method not found",
 		OrderStatusError:              "Order status error",
 		InsufficientOfPeriod:          "Insufficient number of period",
+		ExistAvailableTraffic:         "存在可用流量",
 		OrderAlreadyRefunded:          "Order already refunded",
 		OrderRefundNoSubscription:     "Refund target subscription not found",
 		OrderRefundCommissionMismatch: "Refund commission source not found",