Verifies one factor against the challenge issued by /user/passkey/delete and removes the credential that challenge was bound to. — Required body fields per factor: sms needs code (6-digit numeric); totp needs code (6 numeric digits or XXXXX-XXXXX recovery code); passkey needs origin and credential (the assertion produced from the options returned at initiate). — One challenge authorises one deletion: confirming an already-consumed challenge is rejected with 999998; an unknown or expired challengeId, or a factor the challenge did not offer, with 999995; a wrong code with 999997. The attempt cap is the same as for transaction challenges. If the proof is accepted but the removal fails downstream, the challenge is handed back and the same challengeId can be confirmed again with a fresh proof.