Reports whether the Service Bus namespace is reachable (optional, M70c).

The real adapter peeks the namespace via its existing client; a transport without a liveness check omits it and the broker reports unknown reachability. The SDK owns streaming-pull reconnection, so the broker issues no reconnect loop of its own.

Resolving undefined means could not determine — the probe itself failed rather than the namespace answering (V5-2). Widened from Promise<boolean> in 0.5.1; an implementation that still resolves a plain boolean satisfies it unchanged.