Add FlapperState class and per-client exponential backoff tracking to
MqttSessionStore. New TrackConnectDisconnect(string) overload returns
FlapperState with backoff level and expiry; IsFlapper, GetBackoffMs,
ClearFlapperState, and CheckAndClearStableClients give callers full
visibility and cleanup control. Legacy two-arg overload preserved for
backward compatibility. Ten unit tests cover counting, threshold,
exponential growth, 60s cap, window reset, and stable-client sweep.
5fea08dda0