feat(python): Add Polygon Amoy network support

[AbrahamOjes]

Summary

This PR adds Polygon Amoy testnet support to the Python x402 SDK, bringing it to parity with the TypeScript implementation.

Changes

networks.py

• Added polygon-amoy to the SupportedNetworks Literal type
• Added chain ID mapping: polygon-amoy: 80002 to EVM_NETWORK_TO_CHAIN_ID

chains.py

• Added polygon-amoy: "80002" to NETWORK_TO_ID mapping
• Added USDC token configuration for chain ID 80002:
  • Address: 0x41E94Eb019C0762f9Bfcf9Fb1E58725BfB0e7582
  • Name: USDC
  • Decimals: 6
  • Version: 2

Tests

• Added comprehensive test suite in test_polygon_amoy.py covering:
  • Network support verification
  • Chain ID mappings
  • USDC token configuration
  • Utility function compatibility

Testing

All existing tests pass (87 tests) plus 6 new tests for Polygon Amoy support:

tests/test_polygon_amoy.py::test_polygon_amoy_supported_network PASSED
tests/test_polygon_amoy.py::test_polygon_amoy_chain_id_mapping PASSED
tests/test_polygon_amoy.py::test_polygon_amoy_known_tokens PASSED
tests/test_polygon_amoy.py::test_get_chain_id_polygon_amoy PASSED
tests/test_polygon_amoy.py::test_get_default_token_address_polygon_amoy PASSED
tests/test_polygon_amoy.py::test_get_chain_id_direct_chain_id PASSED

Fixes #670

[cb-heimdall]

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