feat: align v2 with spec (@.chainId path and metadata.contractName)

[fsamier]

Summary

Aligns the v2 implementation with the ERC-7730 spec on two points.

1. Implement @.chainId container path

The spec lists @.chainId as a canonical container field (for both EVM transaction and EIP-712 containers), notably used as a map keyPath (e.g. underlyingToken keyed by chain id). This adds full support for it:

• Added CHAIN_ID = "chainId" to ContainerField.
• Added chainId to the path parser grammar.
• Added CHAIN_ID = 0x3 to the calldata binary value enum and handled it in convert_container_path (encoded as a 32-byte UINT).
• Added parsing tests for @.chainId.

2. Use metadata.contractName for the contract name

Prefer the explicit metadata.contractName field where a contract name is set, falling back to the previous value when not available:

• Calldata v2 conversion: metadata.contractName or context.id.
• Legacy EIP-712 v2 conversion: InputEIP712Contract.contractName prefers metadata.contractName, falling back to metadata.owner (dapp name keeps its owner-based fallback).
• v2 max-length lint validates the effective contract name (metadata.contractName or context.id).

v1 paths are left untouched since v1 metadata has no contractName field.

Testing

• pdm run pytest tests/model tests/convert tests/v2 tests/lint — all passing.
• Pre-commit hooks (ruff, mypy, bandit, etc.) pass.