Function create_layer3_checkout 

pub fn create_layer3_checkout(
    serialized_l2: &str,
    mandate: &CheckoutL3Mandate,
    agent_key: &EcdsaKeyPair,
    agent_kid: &str,
    iat: i64,
    exp: i64,
) -> Result<L3CheckoutResult, ViError>

Create an L3b checkout mandate signed by the agent’s key.

agent_kid names the key in the L2 mandate’s cnf.jwk.kid, on the same rule as create_layer3_payment.