Add x402-middy package for AWS Lambda integration

[junbeomlee]

Implement Middy middleware for AWS Lambda functions following x402 Payment Protocol:

• Support both EVM and Solana (SVM) networks
• Payment verification and settlement flow
• Coinbase Onramp session token integration
• Comprehensive test coverage (29 tests)
• Follow x402-express conventions

Description

This PR adds a new x402-middy package that provides Middy middleware for integrating the x402 Payment Protocol with AWS Lambda functions.

The implementation follows the same conventions and patterns as the existing x402-express package, adapted for the AWS Lambda/Middy ecosystem.

Key Features:

• Middy middleware for AWS Lambda functions (APIGatewayProxyEvent/APIGatewayProxyResult)
• Support for both EVM networks (Ethereum, Base, etc.) and Solana networks (SVM)
• Complete payment verification and settlement flow using x402 facilitator
• Optional Coinbase Onramp integration for session token generation
• Comprehensive documentation with Serverless Framework and AWS SAM examples

Files Added:

• typescript/packages/x402-middy/src/index.ts - Main Middy middleware
• typescript/packages/x402-middy/src/session-token.ts - Lambda handler for Coinbase Onramp session tokens
• typescript/packages/x402-middy/src/index.test.ts - Middleware tests (15 tests)
• typescript/packages/x402-middy/src/session-token.test.ts - Session token tests (14 tests)
• typescript/packages/x402-middy/README.md - Complete documentation with examples

Tests

All tests pass (29/29):

$ cd typescript/packages/x402-middy && pnpm test

 ✓ src/session-token.test.ts (14 tests)
 ✓ src/index.test.ts (15 tests)

 Test Files  2 passed (2)
      Tests  29 passed (29)

Test Coverage:

• Payment verification and settlement flow
• Error handling (verification errors, settlement failures)
• EVM and Solana network support
• Session token integration
• Browser paywall HTML rendering
• Edge cases (missing headers, invalid payments, unsuccessful settlements)

Checklist

• [x] I have formatted and linted my code
• [x] All new and existing tests pass
• [x] My commits are signed (required for merge)

[cb-heimdall]

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