Feat(x402): add payment protocol support

[Benjpoirier]

Description

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 LinkupX402Signer protocol and create_x402_signer helper that
  wires up x402[httpx,evm] for Base chain EVM payments
• Add LinkupPaymentRequiredError for cases where a 402 is received but cannot
  be handled (e.g. no signer configured, signing failure)
• Merge base headers (User-Agent, Authorization) with payment headers on
  retry, matching JS SDK behavior

Usage

  from linkup import LinkupClient
  from linkup.x402 import create_x402_signer

  signer = create_x402_signer(private_key="0x...")
  client = LinkupClient(x402_signer=signer)

  result = client.search(
      query="What is x402?",
      depth="standard",
      output_type="sourcedAnswer",
  )

Checklist

• I have installed pre-commit on this project (for instance with the make install-dev command)
  before creating any commit, or I have run successfully the make format-lint command on my
  changes.
• I have run successfully the make test command on my changes.
• I have updated the README.md if my changes affected it.

[cjumel]

Looks almost good! Just left very small and not important comments, one more important one is: since we've setup the CI with automatic release with semantic versioning and I assume that you're going to squash & merge this branch, you should make sure that the PR title follows the conventional commit convention, so that it's parsed as expected 🙂

[cjumel]

See my previous comment

[cjumel]

✅