Function encode_connection_validation_client_ca 

pub fn encode_connection_validation_client_ca(
    buffer_size: u32,
    introspection_registry_size: u16,
    qos: u16,
    method: &str,
    user: &str,
    host: &str,
    is_be: bool,
) -> Vec<u8> 

Encode just the payload of a client->server CONNECTION_VALIDATION message carrying a “ca”-style auth method plus its trailing (user, host) credentials PVStructure. Returns payload bytes only (no PVA header) — suitable for feeding directly to PvaConnectionValidationPayload::new.