Adds a thin getter over the embedded NATS server's NumLeafNodes(). Consumers that want to implement a "hub auto-shuts down when the last leaf disconnects" lifecycle can poll this — that's the immediate use case from the sesh wrapper spike, where the hub spawns on first session and exits on last.

Three lines, no behavior change.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> check-in: 4caf310202 user: [email protected] tags: trunk