Add OPG token Permit2 approval utilities for x402 payments

[adambalogh]

Summary

This PR adds utilities for managing OPG token Permit2 approvals on Base mainnet, enabling seamless integration with x402 payment infrastructure. The new module provides a high-level API for ensuring sufficient allowances with minimal transaction overhead.

Key Changes

• New opgToken.ts module with Permit2 approval management:

  • ensureOpgApproval() - Main function that checks and maintains minimum Permit2 allowance thresholds
  • Helper functions for reading allowances, balances, and sending approval transactions
  • Utility functions for converting between OPG decimal amounts and base units

• Exported public API:

  • ensureOpgApproval() - Idempotent approval function for backend services
  • BASE_OPG_ADDRESS - OPG token contract address on Base
  • BASE_MAINNET_RPC - Base mainnet RPC endpoint (configurable via env var)
  • Permit2ApprovalResult - Type for approval operation results

• Updated index.ts to export new OPG approval utilities

Notable Implementation Details

• Smart approval buffering: Defaults to approving 2x the minimum threshold to reduce transaction frequency across service restarts
• Robust confirmation handling: Polls allowance state after transaction confirmation to ensure visibility before returning
• Balance validation: Checks wallet balance before approval and caps approval amount if needed
• Precision-safe conversion: Uses string arithmetic for OPG amount conversions to avoid floating-point precision loss
• Configurable RPC: Supports BASE_MAINNET_RPC environment variable for custom RPC endpoints
• Comprehensive error handling: Provides clear error messages for insufficient balance, invalid amounts, and transaction failures