HandleSetStaticAttribute was made fire-and-forget (commit 2951507) — it no
longer replies with SetStaticAttributeResponse — but three InstanceActor
tests still ExpectMsg<SetStaticAttributeResponse> and timed out. Verify the
mutation via the GetAttributeRequest round-trip instead, which the FIFO
mailbox makes a sound sync point. Test intent (in-memory update, SQLite
persistence, serialized ordering) is unchanged.
ClusterClient Sender refs are temporary proxies — valid for immediate reply
but not durable for future Tells. Events now flow as DebugStreamEvent through
SiteCommunicationActor → ClusterClient → CentralCommunicationActor → bridge
actor (same pattern as health reports). Also fix DebugStreamHub to use
IHubContext for long-lived callbacks instead of transient hub instance.
