natsmsg: trace-context propagation, KeepInProgress, FakeMsg test double

Unify entire-core's internal/natsmsg and entire-api's internal/otelnats (itself ported from mirror-pipeline) into one shared package: the HeaderCarrier propagator adapter with Inject/Extract/ExtractHeader and the StartConsumerSpan prologue, the capped KeepInProgress AckWait heartbeat for long handlers, and natsmsgtest.FakeMsg so consumers can assert message disposition without a broker.

First package to pull the OpenTelemetry API and nats.go/jetstream into the module (the dependency decision flagged on COR-929); the root package stays nats.go-only, and subpackage isolation keeps these out of root-only importers.