GcpPubSubBroker.prototype.reachability(): Promise<boolean | undefined>
Tri-state backend reachability (M70c).
The GCP SDK owns streaming-pull reconnection, so the broker issues no
reconnect loop of its own; the probe delegates to the transport's
isHealthy?() (the real adapter calls the SDK's topic.exists()).
true/false from the transport, undefined when the transport omits
the member (a minimal fake) — the indicator then reports
reachable: 'unknown'.