Upgrade x402 from v1 to v2

[racket2000]

Summary

Migrates all four x402 touchpoints — the Python agentic client, both seller Lambdas, and the React frontend — to the x402 v2 wire format, and unpins the Python SDK (x402>=0.1.0,<1.0 →x402[httpx,evm]>=2.0.0,<3.0). The old pin
existed because v2 broke the x402.clients.httpx imports; this PR adopts the v2 API instead.

Wire format changes (applied consistently everywhere)

• x402Version 1 → 2
• maxAmountRequired → amount
• network "base-sepolia" → CAIP-2 "eip155:84532"
• resource/description/mimeType hoisted to a top-level resource object
  on the 402 body
• Payment payload now carries the full accepted requirements object (the
  live facilitator rejects flat scheme/network; the current TS SDK schema is
  stale on this)
• PAYMENT-SIGNATURE only — dropped the v1 X-PAYMENT fallback (CORS updated
  in CDK too)

Python client (agentic/wallet.py)

• Rewrote get_x402_httpx_client to the v2 API:
  x402Client().register("eip155:*", ExactEvmScheme(signer)) +
  register_policy(prefer_network/prefer_scheme), wrapped by
  x402HttpxClient(client, base_url=...)
• Replaced the 3-arg signing adapter with _CdpWalletSigner implementing the
  v2 ClientEvmSigner protocol (4-arg sign_typed_data returning raw bytes)
• Added to_caip2() network mapping

Sellers (serverless/lambda/seller/seller.js, agentic/lambda/seller.js)

• Build and send the v2 accepted object on both /verify and /settle
• Point directly at the canonical www.x402.org/facilitator — the apex
  x402.org 308-redirects every call to www, so this avoids a redirect
  round-trip per payment. The existing redirect handlers stay as a harmless
  fallback.
• Seller remains the source of truth for the authorized amount

Verification

Tested end-to-end against the live facilitator:

• Signed EIP-712 v2 round-trip recovers the correct payer
  (insufficient_balance on a throwaway key = format correct)
• Both sellers run locally: serverless through verify → generate → settle with
  a valid PAYMENT-RESPONSE header; agentic through deferred settle-by-nonce
• Confirmed sellers hit www.x402.org with a single 200 (no redirect hop)
• Frontend tsc --noEmit clean and vite build succeeds