Adds 97 tests across two new files covering stream replication semantics
(R1/R3 creation, replica group size, publish preservation, state accuracy,
purge, update, delete, max limits, subjects, wildcards, storage type) and
placement semantics (replica caps at cluster size, various cluster sizes,
concurrent creation, stepdown resilience, long names, re-create after delete).
Adds a unified JetStreamClusterFixture consolidating the capabilities of all 7
per-suite fixtures (ClusterFormationFixture, ClusterStreamFixture, LeaderFailoverFixture, etc.)
into a single reusable helper for Tasks 6-10. Includes new Go-parity helpers
(WaitOnStreamLeaderAsync, WaitOnConsumerLeaderAsync, GetConsumerLeaderId,
StepDownMetaLeader, SimulateNodeRestart, RemoveNode) matching jetstream_helpers_test.go.
27 smoke tests verify all capabilities pass.
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.