Function build_ack 

pub fn build_ack(
    header: &InboundHeader,
    code: AckCode,
    text: Option<&str>,
    timestamp: &str,
) -> String

Builds an ACK message replying to header.

Addressing is reversed – the sender’s application and facility become the receiver’s – so the reply routes back the way the message came. text is placed in MSA-3 and is what an operator reads when a message is rejected, so it should say what was wrong rather than that something was.