Feat(developer-sdk:typescript): x402 Swig Transaction Preparation

[thewuhxyz]

Summary

Adds x402 exact-Solana payment preparation to the TypeScript Developer SDK.

Developers can pass a resource server’s 402 Payment Required response directly to a Swig wallet:

const prepared = await wallet.x402.prepareFromResponse(response, {
  acceptedIndex,
});

When acceptedIndex is omitted, transaction-service selects the first eligible requirement and returns its original index.

Changes

• Validate PAYMENT-REQUIRED using the pinned @x402/core V2 schema.
• Add x402 preparation to server and browser wallet handles.
• Add the /x402/prepare browser-proxy route.
• Use the existing requester-authority resolution behavior.
• Normalize the x402 prepared-transaction kind and signature requests.
• Reuse the generic prepared-transaction signing helpers.
• Add createX402Payment to assemble:
  • The x402 PaymentPayload
  • A Record<"PAYMENT-SIGNATURE", string> header
• Preserve supported x402 extension values and property presence.
• Validate transaction encoding, network, wire size, and outbound header size.
• Add SDK documentation and a minor-version changeset.
• Pin @x402/core to 2.21.0.

Validation

• 116 Developer SDK tests passed.
• TypeScript typecheck passed.
• ESLint passed.
• Package build passed.
• Prettier and git diff --check passed.

Scope

This PR contains only TypeScript Developer SDK changes.