Function build_inbound 

pub fn build_inbound(
    from: &str,
    to: &str,
    subject: &str,
    size: usize,
    sender_verified: bool,
    sender_did: Option<String>,
) -> InboundMessage

Construct an InboundMessage envelope for the rules engine.

This is pure: it only populates the struct from its arguments — no I/O, no clock. It is the single place both real IMAP delivery and tests build the envelope the rules engine consumes, so the mapping from wire data to the rules’ view lives in exactly one spot.