VIES placeholder address propagates to order and breaks delivery (FI '00000 OSOITE TUNTEMATON')

Bug: When VIES returns a placeholder address for a valid VAT number (e.g. the Finnish 00000 OSOITE TUNTEMATON when no address is published), revalidateVat() copies it straight onto the billing address. The order then has an empty/invalid postcode+city and Ingrid rejects it (On Hold). Seen on order #13467 (FI, returning customer).

Detection is country-agnostic: we don't enumerate per-country placeholder strings - we check whether the resulting postcode/city is usable (empty or all-zero postcode, or empty city = unusable).

Gate: only valid VAT numbers reach the propagation path; invalid VAT already propagates nothing.

Two use cases (valid VAT + unusable VIES address):

A. Logged-in / registered customer: fall back to the customer's registered billing_default address (their real address on file); keep company name + VAT.
B. Anonymous checkout: fall back to the delivery address entered at checkout; keep company name + VAT; let the order proceed.

Fix: PR #593 - VatNumber::hasUsableAddress() + revalidateVat() uses the VIES address when usable, otherwise the customer's real address per the two cases above. Unit tests for the detection.

Pull requests