Port the main payment processing functionality

Tasks:
Payment Middleware Class

• createPaymentMiddleware() function to Python class
• Implement request price calculation interface
• Handle wallet integration and validation

Payment Flow Implementation

• Implement 402 Payment Required response logic
• Port BSV payment header parsing and validation
• Handle derivation prefix creation and verification

Transaction Processing

• wallet.internalizeAction() (Depends on WalletToolbox work below)
• Implement payment result handling
• Add proper error handling and status codes