test(python/evm): add unit tests for FacilitatorWeb3Signer methods

python/x402/changelog.d/facilitator-web3-signer-method-tests.doc.md

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+Add 17 unit tests for `FacilitatorWeb3Signer` functional methods in `mechanisms/evm/signers.py`. The existing `test_signer.py` only covered constructor, address property, and protocol surface; the actual behavior of the on-chain methods was untested. New tests cover `get_chain_id` (RPC call + caching), `read_contract` (address checksumming + `{"from": signer.address}` propagation), `verify_typed_data` (EOA happy-path, mismatched address with no contract code, `bytes` nonce normalization without mutating caller's message dict, raw-dict domain for Permit2-style types, and exception swallowing), `write_contract` (build → sign → send raw with checksummed contract address and propagated nonce/gas/gasPrice), `send_transaction` (raw tx build with checksummed `to`, account signing, hex tx-hash return), `wait_for_transaction_receipt` (status=1 → success, non-1 → failed, missing `0x` prefix normalized in both web3 call and returned receipt), `get_balance` (zero-address → native, empty-string token → native, real token → ERC20 contract path with checksummed addresses), and `get_code` (checksumming + `bytes()` wrapping for both contract and EOA cases).