test(python/svm): add unit tests for utils helpers

[0xAxiom]

Summary

Add 54 unit tests for python/x402/x402/mechanisms/svm/utils.py helpers without prior dedicated coverage. test_index.py already covers the public re-exports (normalize_network, validate_svm_address, get_usdc_address, convert_to_token_amount); the new test_utils.py covers the remaining helpers:

• get_network_config — CAIP-2 (mainnet/devnet/testnet) + V1 names (solana, solana-devnet, solana-testnet) + unsupported-network errors (unknown CAIP-2, unknown V1 name, non-Solana CAIP-2 like eip155:1).
• get_asset_info — default-asset short-circuit on None/empty string, registered-address match, mismatched-address ValueError, network-error precedence over asset matching.
• parse_amount + format_amount — multi-decimal arithmetic (6/9/0 decimals), sub-unit truncation, zero, round-trip format_amount(parse_amount(s, d), d) == s for representative inputs, decimal.InvalidOperation for invalid strings.
• parse_money_to_decimal — int/float pass-through, $ prefix, USD/USDC and usd/usdc suffixes, surrounding whitespace, invalid string raises ValueError.
• decode_transaction_from_payload — valid base64 round-trip (build a real VersionedTransaction, encode, decode, assert bytes(decoded) == tx_bytes); invalid_exact_svm_payload_transaction raised for invalid-base64, base64-garbage, and empty payload.
• derive_ata — valid base58 output (Pubkey.from_string round-trip), determinism across calls, omitted token_program defaults to Token Program, Token-2022 program produces a different ATA, ATA differs across owners and across mints, explicit None for token_program matches default behavior.

All 54 tests pass on uv run pytest; uvx ruff format + uvx ruff check clean.

Test plan

• uvx ruff format tests/unit/mechanisms/svm/test_utils.py — 1 file unchanged
• uvx ruff check tests/unit/mechanisms/svm/test_utils.py — All checks passed
• uv run pytest tests/unit/mechanisms/svm/test_utils.py -v — 54 passed
• Changelog fragment added: python/x402/changelog.d/python-svm-utils-tests.doc.md
• Commit GPG-signed

[cb-heimdall]

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