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.
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.
