Handle 5-byte Modbus exception frames without false “malformed packet” errors in state 7

[Copilot]

A single-tower LVS setup can return a 5-byte Modbus exception response during state 7 polling, which was being reported as Invalid CRC or malformed packet ... (len=5). This PR treats that frame as a valid protocol exception instead of a malformed packet.

• Protocol-aware exception frame handling

  • Added _is_modbus_exception_response(data, request) to recognize valid Modbus exception packets (address, function|0x80, exception_code, crc).
  • Added focused constants for exception parsing (MODBUS_EXCEPTION_PACKET_LENGTH, MIN_MODBUS_REQUEST_LENGTH, MODBUS_EXCEPTION_FLAG).

• State read-path behavior update

  • In _send_and_receive(...), when a valid exception frame is detected:
    • log as a Modbus exception warning (including tower/state context),
    • stop the current polling cycle cleanly.
  • Keep existing malformed/invalid CRC logging for packets that do not match valid exception framing.

• Guardrails for false positives

  • Exception detection now validates request/response address alignment before matching function-code exception flag.

if data and self._is_modbus_exception_response(data, request):
    exception_code = data[2]
    _LOGGER.warning(
        "Modbus exception response in %s for tower %d (exception=0x%02X, len=%d)",
        state_name, self.current_tower, exception_code, len(data),
    )
    self._state = 0
    return None