Migrate x402 MCP integration from legacy x402 package to @x402/core and @x402/evm v2 #793

Summary

Update packages/agents/src/mcp/x402.ts to use the new x402 v2 packages (@x402/core, @x402/evm) instead of the legacy x402 package.

Background

The x402 protocol has been refactored into a modular architecture. Key changes in v2:

See: Migration Guide (https://github.com/coinbase/x402/blob/main/docs/guides/migration-v1-to-v2.md)

Motivation

1. The legacy x402 package is being deprecated
2. v2 architecture enables future Solana support (@x402/svm)
3. CAIP-2 network identifiers are the standard for multi-chain compatibility

Requirements

• Migrate to @x402/core and @x402/evm packages
• Update server-side (withX402) and client-side (withX402Client) implementations to v2 APIs
• Support both legacy network names and CAIP-2 format for backward compatibility
• Update examples/x402-mcp to work with v2
• No breaking changes to public API types