Enable Redis Cluster initialization for Java client

[zhchxiao123]

What to build

Make the Java SDK's common client initialization path honor Redis Cluster configuration so applications using the standard by client entry point can run against Redis Cluster without accidentally creating a standalone connection. The behavior should keep standalone mode as the default, while allowing Cluster mode to be selected through the existing Redis connection configuration and requiring the v2 key schema guard that prevents unsupported Cluster key layouts.

Acceptance criteria

• The Java client's common singleton/default initialization path can create a Cluster-backed Redis client from Redis mode, Cluster node, auth, timeout, and key schema configuration.
• Standalone initialization remains backward compatible for existing gateway.redis.* configuration users.
• Cluster mode still fails fast unless REDIS_KEY_SCHEMA_VERSION=v2 is configured.
• Tests cover standalone default initialization, Cluster configuration parsing, and the v2 schema guard.
• Documentation or examples show the supported Cluster initialization path for Java by client users.

Blocked by

None - can start immediately