Add x402Version field to all protocol messages

[andreabadesso]

Current behavior

Our 402 response does not include an x402Version field:

{
  "accepts": [...],
  "version": "1"
}

The version field is our own; it's not the x402 protocol version.

Required behavior

Add x402Version: 2 to:

1. 402 response (PAYMENT-REQUIRED): Top-level field in the response body
2. PAYMENT-SIGNATURE header: Include in the payment payload
3. PAYMENT-RESPONSE header: Include in the settlement result

{
  "x402Version": 2,
  "accepts": [...],
  "version": "1"
}

Changes needed

Resource server (resource-server.js)

• Add x402Version: 2 to the 402 JSON response

Client (client.js) and dApp (dapp/components/X402Fetch.tsx)

• Include x402Version: 2 in the payment payload

Facilitator (facilitator.js)

• Include x402Version: 2 in verify/settle responses

References

• x402 V2 Migration Guide