Tri-state backend reachability (M70c, bounded in M90b).

The Azure 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 reads the namespace through the administration client — through the broker's own cached probe (5 s TTL, 2 s bound), so repeated health polls cost at most one round trip per TTL and a hung transport cannot hold a caller past the bound. true/false from the probe, undefined when the transport omits the member (a minimal fake) — the indicator then reports reachable: 'unknown'.