UpdateTagPayload

The result of updateTag: the tag as it now stands, or the reason the edit was refused.

Members on the way out. RestProxyError, NotFoundError, UnauthorizedError and UnexpectedError are being removed from this union. No new operation should select them by name.

RestProxyError is never returned. The other three describe situations a caller cannot correct — the tag not existing, the caller not being permitted, an unexpected fault — and will be reported in the response's top-level errors array instead of here.

Select ... on MutationError { message } rather than naming concrete error types. Every error member implements that interface, so the selection keeps working across the change.