Attaches a sign-in credential to the calling account.

The caller is the account being upgraded, so the request carries no user ID and the session is the authorization. The call is safe to retry: each field is compared against the account first, and a field the account already holds is dropped rather than rewritten.

The moderation filter is bypassed because an account held by a birthday compliance restriction is Suppressed and lifts that restriction through this call. Every upgrade type still passes its own eligibility gate.