test(python): add unit tests for sign_eip2612_permit

[VedantAnand17]

Summary

The Python eip2612_gas_sponsoring extension's sign_eip2612_permit client function had no direct unit coverage — only the surrounding declaration / serialization / extraction / payment-validation paths were tested. This mirrors the testing approach #2278 took for sign_erc20_approval_transaction in the sibling erc20_approval_gas_sponsoring extension.

No production code changes; tests-only.

Changes

• python/x402/tests/unit/extensions/test_eip2612_gas_sponsoring.py: add a _StubPermitSigner (records every read_contract and sign_typed_data call) plus a TestSignEip2612Permit class with five focused tests covering:
  • returned Eip2612GasSponsoringInfo shape (all fields populated, signature hex-prefixed),
  • nonce read against the token contract via EIP2612_NONCES_ABI with the signer address,
  • EIP-712 domain (name, version, chainId, verifyingContract) and Permit primary type,
  • message int coercion (value, nonce, deadline) plus correct owner/spender (Permit2),
  • MaxUint256 amount support.
• Towncrier misc fragment under python/x402/changelog.d/.

Why

sign_eip2612_permit constructs the EIP-712 payload that downstream facilitators verify on-chain. A regression in the domain, the Permit primary type, the int-coerced message fields, or the spender (must be PERMIT2_ADDRESS) would silently break Permit2 gas sponsoring on every chain that opts into it. Tests now lock those invariants in.

python/CONTRIBUTING.md calls this out:

MATCH EXISTING PATTERNS: TypeScript SDK uses pattern X → Python SDK MUST use pattern X.

The TypeScript and Go sides of the EIP-2612 extension on feat/eip2612-extension-implementations already ship dedicated unit tests for their permit-signing paths; this brings Python to parity for the function that already lives in main.

Test plan

• uv run pytest x402/tests/unit/extensions/test_eip2612_gas_sponsoring.py — 18 passed (5 new).
• Full Python suite: uv run pytest — 1140 passed, 45 skipped.
• uv run ruff check — clean.
• uv run ruff format --check — 234 files already formatted.
• No production code touched: tests + changelog fragment only.

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[VedantAnand17]

@Must-be-Ash @phdargen can you please check this out :)