Add ILeaderForwarder interface and DefaultLeaderForwarder, update
JetStreamApiRouter with RouteAsync + ForwardedCount so non-leader nodes
can attempt to forward mutating requests to the meta-group leader before
falling back to a NotLeader error response.
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.