charts: index on a nil _cluster fails the render before the cluster-domain default applies #4046

Twelve templates across clickhouse, foundationdb, kubernetes, mongodb, nats, opensearch and qdrant read the cluster domain as

{{- $clusterDomain := (index .Values._cluster "cluster-domain") | default "cozy.local" }}

index on a nil map is a hard render error, so the default never runs when _cluster is absent: helm template without the platform-injected _cluster block fails with index of untyped nil instead of falling back to cozy.local. On a platform install _cluster is always injected through the values Secret, so the failure surfaces in local renders and in helm-unittest suites that do not inject the key, not in production. mariadb already guards it the way the rest should:

{{- $cluster := .Values._cluster | default (dict) }}
{{- $clusterDomain := (index $cluster "cluster-domain") | default "cozy.local" }}

One sweep over the twelve reads, plus a unit case per chart that renders without _cluster, closes it.