Health contract: an MTConnectError document (NO_DEVICE, INVALID_REQUEST, ...) is the
Agent answering. The parsers now mark such an InvalidDataException
(MTConnectXml.IsAgentRefusal - the type is sealed, so a Data marker, every other
handler unchanged); ReadAsync reports the batch Bad but does not degrade the read
path, advances LastSuccessfulRead, clears a prior read degradation and warns once
per distinct refusal. An unparseable answer stays a comms fault.
Refs #743
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
ac01c88c0e