ab_server's RegisterSession sanity check rejects any non-zero sender context
with encapsulation status 1 and a header-only reply, so every native session
against the simulator failed with BadCommunicationError. libplctag sends zero
(logix-register-session.hex); the codec test now matches that capture byte for
byte instead of excluding the context, and a client test pins the zero on the wire.
Joseph Doherty