List offerable number countries

Deprecated alias of /v1/phone-numbers/countries; same contract. New integrations should use that path.

The WhatsApp number countries available to purchase, each with its flat monthly price (cents), regulatory tier, whether it needs end-user KYC (Tier 3/4), and whether outbound calling is available (not BIC-blocked). Drives the country picker. Tier-4 countries appear only when enabled.