Add DeliverWill to MqttPacketHandlers — extracts and clears the will,
then delivers via ProcessInboundClientMsg. Call from CloseConnection
when client is MQTT. Clean DISCONNECT already clears the will (Task 3),
so DeliverWill is a no-op after DISCONNECT. 3 new tests: abnormal
close delivers will, DISCONNECT suppresses will, no-will is a no-op.