fix(cluster): fall back to origin port when CLUSTER SLOTS reports port 0 (#3828)

In TLS-only clusters started with `--port 0 --tls-port <port>`, CLUSTER
SLOTS reports the regular port as 0. newClusterState used to pass that
value verbatim into clusterNode.Client.opt.Addr, which the PubSub path
dials directly with no MOVED/seed fallback, producing
`dial tcp <ip>:0: connection refused`.

Add a replaceZeroPort helper symmetric to replaceLoopbackHost: when a
slot node's port is "0", substitute the origin endpoint's port — by
definition reachable, since it is the port we used to fetch the slot
map. NodeAddress is left untouched so maintenance-notification matching
continues to see the server-reported value.

Adds TestNewClusterState_PortZero covering the bug scenario plus the
non-zero passthrough and the loopback+:0 composition.

Fixes #3726