The PreserveReplicaOrderIterator in TokenAwarePolicy previously returned
only replica nodes in the query plan. When replicas were unavailable
(e.g. prepared statements before parameter binding, or replicas going
down after plan construction), the query plan could be empty or
insufficient, causing "No node was available" errors.

The fix adds a third pass that appends non-replica nodes from the child
policy after all replicas have been returned. When no replicas are
available at all (all DOWN/IGNORED), the full child policy fallback is
preserved.

Fixes #833