x402 server-side middleware: respond to 402 with switchboard-managed payments

[abhicris]

Why

x402 joined the Linux Foundation on 2026-04-02 with Coinbase, Google, AWS, Microsoft, Stripe, Visa, Mastercard, Circle as founding members and 35M+ transactions on Solana since launch. It's becoming the default agent ↔ HTTP-server payment standard. Switchboard already manages agent wallets, nonces, and gas budgets — it should also be the easiest way for a switchboard-managed agent to respond to an x402 challenge, and (separately) for a service to issue x402 challenges paid out of switchboard escrow.

Scope

Two pieces, behind feature flags:

1. Client middleware (switchboard.x402.client) — wraps any requests / httpx / fetch call. On a 402, parses PaymentRequirements, picks a matching (scheme, network, asset) we have a wallet for, signs a PaymentPayload, retries with PAYMENT-SIGNATURE header. Honours the gas-budget tracker (PR feat: gas budget tracker with configurable hour/day limits (closes #5) #14) — refuses to pay if the wallet is paused.
2. Server middleware (switchboard.x402.server) — Flask + FastAPI + Express adapters that:
   • return 402 + PaymentRequirements when no payment header is present;
   • verify the inbound PaymentPayload (locally for off-chain schemes, on-chain for the exact scheme);
   • emit a receipt + nonce so retries are idempotent.

Deliverables

• switchboard/x402/client.py + switchboard/x402/server.py.
• Sample app under examples/x402_premium_endpoint/ showing a paid /inference endpoint and a switchboard-driven client.
• pytest fixtures with a stubbed facilitator (no live RPC needed in CI).
• Schema-conformance tests against the x402 spec.
• Docs section referencing the /switchboard/ dashboard's protocol map.

Non-goals

• Multi-network selection logic — start with USDC on Base only.
• Fiat / SPT integration (that's MPP territory; tracked separately).