The updated user.

Missing or invalid HTTP Basic credentials.

Resource not found.

The request couldn't be completed. A validation_error is something the
caller can fix — a bad payload, a merchant that failed validation, or a
processor rejecting a field (e.g. the MID); the reason is in fields,
keyed by a processor-agnostic field name. A processor_error is an
unactionable upstream failure (a processor was unreachable, busy, or
errored); fields is empty and the caller should retry or escalate.
Raw upstream detail is never surfaced here — only a parsed,
processor-neutral reason.