Offer Tx Code Spec
Describes the transaction code a pre-authorized credential offer expects the user to enter, mapped from the issuer's offer metadata into an SDK-native type (so apps never touch EUDI's TxCode).
Obtained from com.dewa.walletsdk.api.oidc.oidc4vci.ReceiveDocument.resolveTxCodeSpec; a null result there means the offer is not pre-authorized and no code is required.