Issues a WebAuthn assertion challenge for the application's API passkey, bound to an existing corporate transfer challengeId (returned by /wallets/send/inter/initiate when the application holds an API passkey and verificationMethod was omitted or passkey; such a challenge offers the passkey only, and fallbackViaResend is false). Run this before submitting the signed credential to /wallets/transaction/confirm (with userId = the application id) or /wallets/corporate/transaction/confirm. The request carries no userId: the subject is the application that signed the request.