Function encode_request 

pub fn encode_request(
    txn_id: u16,
    req: &ClientRequest,
    transport_type: TransportType,
) -> Result<Vec<u8, MAX_ADU_FRAME_LEN>, MbusError>

Encodes a ClientRequest into a complete ADU frame using the given txn_id.

For serial-only function codes (FC 07 / 08 / 0B / 0C / 11) the txn_id is passed through but the serial ADU format does not include it on the wire.