Function validate_canonical_session_start_payload 

pub fn validate_canonical_session_start_payload(
    payload: &SessionStartPayload,
) -> Result<(), MacpError>

Validate the complete canonical SessionStart binding contract.

Mode-independent, and therefore holds the roster non-emptiness rule for every mode. Prefer validate_canonical_session_start_payload_for_mode on any path that knows the mode name; this entry point is retained with its original signature and its original behaviour.