Make required header fields infallible after parsing

[davidsteiner]

Problem

After a message passes validation (ParsedMessage::Valid), required header fields like MsgSeqNum and MsgType are guaranteed to exist.

However, accessing them still returns Result, forcing call sites to handle impossible errors with expect() or propagate errors that can never occur.

Suggestion

The Header type could expose required fields as infallible accessors (e.g., header.msg_seq_num() -> u64) populated during parsing, while keeping the flexible .get() API for optional fields.