bug(cache): standalone mode against a cluster-enabled Redis misses silently #669

Area: cache (Redis backend, #626 / #630)

cache.redis.mode: standalone pointed at a cluster-enabled server degrades silently:

• Keys whose slot the node doesn't own get a MOVED reply.
• The breaker counts MOVED as "server up", so it never opens.
• Lookups for those keys always miss, and their bumps stay deferred, with nothing logged.

This comes from reading refusesWork; that the single client does not follow MOVED is inferred.

Options considered in #626, none taken:

• Refuse at connect when INFO cluster reports cluster_enabled:1. This could also refuse single-endpoint cluster services that work in standalone mode today, such as Dragonfly's emulated cluster mode and possibly ElastiCache Serverless. Both are unverified.
• Treat MOVED as a refusal. The breaker would flap whenever the probe key's slot is local.

Suggested: detect MOVED replies and log once at ERROR ("server is cluster-enabled; set cache.redis.mode: cluster") without opening the breaker. Alternatively, verify the single-endpoint services first, then refuse at connect.