How do I validate Y-tunnus (Finnish company ID) + VIES for EU B2B?

Two Finnish B2B identifiers, two validators:

• Y-tunnus (Yritys- ja yhteisötunnus, Business ID), 7-digit + hyphen + check digit format (e.g. 1234567-8). Identifies a Finnish company in the YTJ (Yritys- ja yhteisötietojärjestelmä) register. Validated against the free YTJ API from PRH (Patentti- ja rekisterihallitus) and Verohallinto (Finnish Tax Administration).
• VAT number / TVA intracommunautaire, format FI + 8 digits (the Y-tunnus without hyphen, check digit included). Validated via the VIES VAT service (EU-wide).

For Magento B2B we:

1. Add Y-tunnus + VAT-number fields to the customer entity (or B2B company if on Adobe Commerce).
2. Validate Y-tunnus against the YTJ API on registration, rejects invalid / dissolved companies, auto-fills company name + address.
3. Validate VIES for cross-border zero-rated B2B sales (intra-EU reverse charge).
4. Apply 0% ALV on B2B intra-EU sales with valid VIES; 25.5% on B2C and FI→FI B2B.
5. Cache validations 30 days, re-validate on any address change.