Convenience wrapper around MarketplacesAuthGateway.loginWithCredentials('admin', ...)
that activates the returned session in one call, for apps building a custom admin
login screen. AuthFailure gains a rate_limited code (HTTP 429, parsed Retry-After)
and an optional status field so 401/403/429 surface distinctly without new plumbing.
No credentials, no client-side comparison, no superadmin concept anywhere in this
package - same code path as any other admin credential login. Inert without a
backend implementing POST {credentialsPath} { login, password, mode: 'admin' }.
Additive, backward-compatible. QR/Yandex flows unmodified.
