Factory building the NATS MsgHdrs used to carry transport headers.

Required alongside client for trace propagation: an injected connection carries no nats module, so the broker has no headers() to call. A lazily-loaded connection supplies its own and needs nothing here.

Subjects the broker may create streamName with when the stream is absent on the server.

There is deliberately NO default and no catch-all: NATS refuses a stream capturing every subject unless it is created with no_ack: true, and no_ack makes every JetStream publish reject unobserved (X28-2). Supplied and the stream is absent → the broker creates it with exactly these subjects. Absent and the stream is absent → startup rejects with JetStreamStreamError, naming the stream and both remedies (create the stream out of band, or supply this option). An existing stream is never touched either way.