Constant SESSION_ID_HEX_MAX_CHARS 

pub const SESSION_ID_HEX_MAX_CHARS: usize = 64;

Maximum length, in lower-hex characters, of a SessionId’s body. Unlike MessageId, a session id is derived by the caller from a process identity (mail4agent-attest::PeerProcess’s (pid, started_at_unix_ms) pair, typically hashed) and handed to this crate already formed, so it has no width this crate gets to fix – this bound is generous enough for a SHA-256 hex digest (64 characters), a reasonable way to derive one.