Create a Router Checkout purchase session

Create an authenticated stateful procurement session for buying an outcome. The session returns option groups for provider, quality, verification, output format, fallback policy, and context-sharing policy. V1 session state is durable in the purchase_sessions table but remains short-lived; paid execution still goes through the normal wallet-backed invocation ledger. Provider options are active, approved, API-backed service listings; non-service listings and recurring subscription listings are not selected as V1 checkout provider steps. Anonymous x402 does not support open-ended option negotiation.