Extract schema-valid JSON from a document image with x402 V2.

Always paid from the first call, with no V1 or free image route. Send base64-encoded PNG, JPEG, or WebP bytes plus a JSON Schema. Decode the PAYMENT-REQUIRED response header, sign the live terms, and retry the identical request with PAYMENT-SIGNATURE. The server verifies first and settles only after producing schema-valid JSON. Image bytes are processed transiently and are not persisted by SchemaSure.