feat(extensions): add discovery OpenAPI document builder

[zdql]

Summary

• Adds discovery, a pure function in @x402/extensions that turns an x402 RoutesConfig into an OpenAPI 3.1 document ready to host at /openapi.json.
• Derives paths, parameters, requestBody, responses[200/402/401], x-payment-info, and SIWX security from the routes map and existing bazaar declarations.
• Exposes DiscoveryOptions, OpenAPIDocument, and their Zod schemas (DiscoveryOptionsSchema, OpenAPIDocumentSchema) for runtime validation.
• Adds a dedicated docs page at docs/extensions/discovery.mdx, a row in the Available Extensions table, and a nav entry in docs.json.

Why

bazaar describes a single endpoint inside a 402 response, which is great for on-demand cataloging but does not give crawlers or AI agents a single URL they can fetch to enumerate everything a server offers. discovery closes that gap by reusing the routes map the server already has, with a one-line call-site in the server's framework (e.g. app.get("/openapi.json", (_req, res) => res.json(discovery(routes, options)))).

What it derives automatically

Source in RouteConfig	Lands in
Route key "POST /analyze"	paths["/analyze"].post
config.description	operation.summary
config.accepts[0].price	operation["x-payment-info"].price
config.extensions.bazaar body schema	requestBody.content["application/json"].schema
config.extensions.bazaar query/path schemas	operation.parameters[]
config.extensions.bazaar.info.output.example	responses["200"] (with inferred schema)
config.extensions["sign-in-with-x"] present	security: [{ siwx: [] }], no x-payment-info

Path params (:id, [id]) are auto-converted to OpenAPI {id} syntax. Every paid operation gets responses["402"]; SIWX-only operations get responses["401"] plus a top-level components.securitySchemes.siwx block.

What the caller must provide

DiscoveryOptions carries document-level metadata that cannot be synthesized from routes:

• info.title / info.version (required)
• info.xGuidance — high-level agent guidance (required; emitted as info["x-guidance"])
• info.description (optional)
• servers (optional)
• protocols (optional, defaults to [{ x402: {} }])

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