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.
