feat(ilp): implement open payments mechanism

[dragosp1011]

Description

This pull request implements ILP Open Payments support for the x402 v2 payment protocol through the Interledger Protocol, following the x402 Open Payments exact scheme specification (#1962).

Closes #1242.

Key Features

Payment Format - The Open Payments exact scheme uses these PaymentRequirements fields:

{
  scheme: "exact",
  network: "ilp:openpayments",             // CAIP-style network identifier
  amount: "100",                           // Amount in smallest asset unit (e.g. USD cents)
  asset: "USD",                            // Asset code from wallet discovery
  payTo: "https://wallet.example.com/alice", // Server wallet address URL
  maxTimeoutSeconds: 60,                   // Payment validity window
  extra: {
    assetScale: 2                          // Decimal places (2 for USD cents)
  }
}

Key Differences from EVM/SVM:

• ILP-based settlement (not on-chain): funds move via Interledger; settle is a no-op on the facilitator
• GNAP grants: client obtains short-lived bearer tokens from the auth server before each operation
• Wallet discovery: server and facilitator resolve asset code, scale, auth server, and resource server from the wallet address URL at runtime
• Incoming payment polling: client creates an incoming payment and polls until completed = true before returning proof
• Proof format: incomingPaymentUrl (string) instead of a transaction hash

Tests

Automated Tests:

pnpm --filter @x402/openpayments test  // 116 unit tests across 5 files

• 116 unit tests across 5 files
• 1 integration test for wallet address discovery

Manual Testing:

• examples/typescript/facilitator/advanced/all_networks.ts - Multi-chain facilitator
• examples/typescript/servers/advanced/all_networks.ts - Multi-chain server
• examples/typescript/clients/advanced/all_networks.ts - Multi-chain client

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (docs-only changes can skip)