feat(payment): buildX402AcceptsFor402 + probe extra.name fix (1.3.2)

[vvillait88]

Summary

Mirrors the python-commerce 1.3.4 cleanup. Two parallel goals:

1. Lift up the boilerplate every Node merchant inlines for emitting x402 challenges (x402Server.buildPaymentRequirements({...} as never) + push the result) so the EIP-712 domain-mismatch trap can't recur.
2. Fix the discovery probe + tests + examples that hardcoded extra: { name: 'USDC' } regardless of network — wrong on Base mainnet (the actual USDC contract returns name() = "USD Coin"), correct on sepolia (name() = "USDC").

New public API

import { buildX402AcceptsFor402 } from '@agent-score/commerce/payment';

const accepts = await buildX402AcceptsFor402(x402Server, {
  network: X402_BASE,
  price: `$${totalUsd}`,
  payTo: process.env.TREASURY_BASE_RECIPIENT!,
  maxTimeoutSeconds: 300,
});

Wraps server.buildPaymentRequirements(...), returns the result as a list of plain wire-shape objects ready for the 402 body's accepts[]. The x402 scheme registered on the server fills in extra from contract metadata: sepolia → name: 'USDC', mainnet → name: 'USD Coin'.

Probe / examples / tests

• src/discovery/probe.ts: sample accept for eip155:8453 now emits extra: { name: 'USD Coin', version: '2' }; sepolia stays 'USDC'.
• examples/multi-rail-merchant.ts: refactored to use buildX402AcceptsFor402(x402Server, ...) instead of the hand-rolled accept.
• examples/api-provider.ts: keeps inline (smallest possible example) with a comment pointing at the helper as the production pattern.
• tests/discovery/probe.test.ts: assertions updated.
• tests/payment/x402_server.test.ts: 3 new tests for the helper (passes resource-config, honours scheme/maxTimeoutSeconds/extensions, returns [] on non-array fallback).

Tests

669 passed, 4 skipped. Lint + typecheck clean.

Doc updates

• README.md + CLAUDE.md gain a buildX402AcceptsFor402 snippet.
• core/docs/integrations/node-commerce.mdx (Mintlify) updated in the companion core PR (#237).

Test plan

• CI green.
• Tag v1.3.2, push tag → npm publish.
• Follow-up: refactor martin-estate/src/routes/purchase.ts to use the new helper (currently calls x402Server.buildPaymentRequirements(...) directly with an as never cast — works, but the helper drops the cast and tightens the type story).

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