Add x402 payment protocol support

[Benjpoirier]

Summary

• Add support for the X402 payment protocol, allowing clients to
  automatically handle HTTP 402 responses by signing and retrying requests with on-chain
  payments
• Introduce X402Signer abstraction and createX402Signer helper that wires up viem +
  @x402/evm + @x402/core for Base chain EVM payments
• Add LinkupPaymentRequiredError for cases where a 402 is received but cannot be
  handled (e.g. missing header, already retried)
• Add Axios response interceptor that transparently decodes payment-required headers,
  signs payment payloads, and retries the original request

Test plan

• Unit tests for createX402Signer (account creation, public client setup, EVM
  signer wiring, chain registration, payload delegation, error propagation) — 100% coverage
• Unit tests for the X402 interceptor flow (402 → sign → retry → success, missing
  header, already-retried guard, non-402 passthrough, network errors)
• Unit tests for LinkupPaymentRequiredError
• Manual end-to-end testing on testnet with a test wallet

Usage

const signer = createX402Signer({ privateKey: '0x...' });
const client = new LinkupClient({ signer });

const result = await client.search({
  query: 'What is x402?',
  depth: 'standard',
  outputType: 'sourcedAnswer',
});

[Shedna]

unknown -> Promise. Can't we do better here or not ?

[Benjpoirier]

We can't type this better without changing the project's moduleResolution.
With "node", TypeScript can't resolve @x402/core subpath exports (@x402/core/types), so we can't import PaymentRequired / PaymentPayload directly. Keeping unknown for now — we'll type it properly when we migrate to moduleResolution: "node16" or "bundler

[Shedna]

I think you missed @x402/core also in devDependencies no ?

[Benjpoirier]

yes

[Shedna]

🎉 This PR is included in version 3.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀