Stable, public classification of why a wallet top-up failed, present only on a failed deposit. Values are: - card_declined: the saved card was declined by the issuer - authentication_required: an off-session charge needed a 3DS challenge no auto-topup can satisfy - auto_topup_exhausted: auto-topup spent its retry budget and gave up - auto_topup_no_payment_method: auto-topup was armed but the org has no saved card