jsconsumer: expose InactiveThreshold and MaxAckPending

Both are hard blockers for migrating the remaining hand-rolled consumers: every consumer on the interest-retention webhook stream (entiredb's permswebhook, mirror-pipeline's forwarder) sets a 72h InactiveThreshold so a decommissioned durable stops pinning messages, and the shared-durable work queues bound their cross-replica in-flight window with MaxAckPending. Plain pass-throughs to jetstream.ConsumerConfig; zero values keep today's behavior.