Inspect x402 endpoint

Fetch any URL and, without paying, decode its x402 402 Payment Required challenge: price (atomic and formatted for known stablecoins), asset, network, payTo, resource, description, timeout and any input schema. Flags MPP and Bazaar discovery extensions. Returns free:true when no payment is needed. SSRF-hardened: public hosts only; private, loopback, link-local and cloud-metadata addresses are rejected.