x402 payment challenge for a paid scan

Returns an HTTP 402 payment-requirements challenge for the paid scan resource (x402 v2). This is the discovery/probe surface: it always 402s with the v2 PaymentRequired delivered in the base64 PAYMENT-REQUIRED response header (and mirrored in the JSON body) — x402Version 2 + resource + accepts[] + Bazaar discovery extensions. To run a scan, POST to this path with a signed PAYMENT-SIGNATURE (x402 v2) or X-PAYMENT (v1) header.