Extended the BecomeLeader() fix to JetStreamClusterFixture,
ClusterFailoverFixture, and LeaderFailoverParityFixture. All three
fixtures now auto-simulate leader election after stepdown, matching
the MetaControllerFixture fix from the previous commit.
Stream lifecycle, publish/ack, consumer delivery, retention policy,
API endpoints, cluster formation, and leader failover tests ported
from Go nats-server reference. 1006 total tests passing.