Cluster fallback threshold in global mode | 集群模式设置集群阈值为N，集群阈值模式为总体阈值，勾选失败退化后阈值使用的集群阈值

流控规则配置成如下：
阈值类型： QPS
集群阈值：100
是否集群： 是
集群阈值模式：总体阈值
失败退化（如果token Server不可用是否退化到单机限流）： 是
token Server 为独立部署，但token Serve宕机后退化成单机模式，就变成了单机限流为100QPS， 跟预期不复合（如果有10个client，单机阈值应该是100/10）。

Start by locating the cluster-threshold fallback path used when the independently deployed token Server is unavailable, and trace how global-mode thresholds are applied in single-machine fallback. Reproduce the configuration with 10 clients, then verify the fallback behavior against the stated expectation that each client receives 100/10 QPS.

Written by the indexing model from the issue text.