Replace the bare TCP-connect probe in ModbusDriverProbe with a two-phase
check: TCP connect via ModbusTcpTransport (keeps the same SocketException /
timeout / generic error paths and messages), then a one-shot FC03 Read
Holding Registers (qty 1 @ addr 0). A normal response → Ok=true "Modbus
FC03 OK"; a Modbus exception PDU → Ok=true "Modbus FC03 OK (device
returned exception PDU)"; any other failure after TCP succeeds → Ok=false
"Reachable at host:port but Modbus FC03 handshake failed: …".
Add ModbusDriverProbeTests (6 tests) covering invalid JSON, missing
host/port, closed port, TCP-accept-then-close, canned MBAP happy path,
and Modbus exception PDU path. All 277 Modbus tests green.
Joseph Doherty
9b909002be