Function format_message_id 

pub fn format_message_id(room: &str, seq: u64) -> String

Format a human-readable message ID from a room ID and sequence number.

The canonical format is "<room>:<seq>", e.g. "agent-room:42". This is a display-only identifier used by --from, --to, and --id flags. The wire format keeps room and seq as separate fields and never stores this string.