fix(billing): 文本模型 - 修复流式 Token 用量结算

为 Token 计费的流式 Chat Completions 请求开启 usage 终态块，并解析 Chat Completions 与 Responses SSE 用量。

backend/internal/handler/auth.go

@@ -715,8 +715,10 @@ func proxySystemRequest(c *gin.Context, svc *service.Service, user *model.User,
 	modelName := proxyRequestModelForPath(path, c.GetHeader("Content-Type"), body)
 	protocol := model.ChannelInterfaceType("")
 	capability := "text"
+	var channelModel *model.ChannelModel
 	if !(c.Request.Method == http.MethodGet && path == "/models") {
-		channelModel, modelErr := svc.SystemChannelModel(channel.ID, modelName)
+		var modelErr error
+		channelModel, modelErr = svc.SystemChannelModel(channel.ID, modelName)
 		if modelErr != nil || channelModel.Protocol == "" {
 			fail(c, http.StatusForbidden, errors.New("当前系统渠道未授权该模型或模型协议尚未配置"))
 			return
@@ -728,6 +730,13 @@ func proxySystemRequest(c *gin.Context, svc *service.Service, user *model.User,
 		fail(c, http.StatusForbidden, err)
 		return
 	}
+	if channelModel != nil && channelModel.BillingMode == "token" && protocol == model.ChannelInterfaceChatCompletion {
+		body, err = service.EnsureChatCompletionStreamUsageRequest(body)
+		if err != nil {
+			fail(c, http.StatusBadRequest, err)
+			return
+		}
+	}
 	billingOrderID := ""
 	query := c.Request.URL.Query()
 	for _, key := range []string{"key", "api_key", "access_token", "token"} {