Get a real, payable Machine Payment Protocol (MPP) URL to purchase an ALREADY-LISTED Atom marketplace domain via Stripe SPT (the SPT-paid sibling of register_domain, which pays from an Atom balance instead) — for use with an MPP-native payment agent (e.g. Stripe's link-cli). Use this ONLY when an MPP-native agent's tools (e.g. link-cli's mpp_pay) are available in this session, and only for domains that are curated Atom listings — for a fresh, unlisted domain, use get_domain_register_pay_link instead.
PAYMENT PRIORITY: this is priority 2 for buying a premium/marketplace domain — reach for it only after register_domain (Atom balance, priority 1) reports insufficient funds, and only when link-cli is present. Always hand over the exact server-computed pay_url/price returned here — never estimate or recompute the amount yourself. If link-cli is not available either, use get_checkout_link (priority 3, last resort) instead.
Returns a pay_url that a real HTTP 402 challenge is served from — hand it directly to the MPP agent's pay tool (e.g. mpp_pay) rather than fetching or decoding it yourself.
If this reports error 'registrant_contact_required' (the transfer needs this to complete once paid), ask the user for their name, phone, address, city, zip, and country, call create_registrant_contact with those, then retry this tool — this is expected and not a dead end.
