When Redis Cluster is configured via the REDIS_CLUSTER_NODES environment
variable, Cache.__init__() and Router._create_redis_cache() ignored the
env var and always created RedisCache instead of RedisClusterCache. This
caused the v3 rate limiter's cluster detection (_is_redis_cluster()) to
return False, skipping hash-slot key grouping. The resulting CROSSLOT
errors were silently caught, falling back to per-instance in-memory
counting — breaking RPM/TPM enforcement across multiple proxy instances.
Add REDIS_CLUSTER_NODES env var detection to both Cache.__init__() and
Router._create_redis_cache(), matching the existing pattern in
_redis.py:215-220. When the env var is set and no explicit startup_nodes
parameter is provided, parse it and create RedisClusterCache.
Fixes#22748
Related to #20836
