fix: Is signatureChainId intentionally hardcoded to 0x66eee in sign_user_signed_action?

[KyaClaw]

Summary

Fix the hardcoded testnet chain ID 0x66eee (Arbitrum Sepolia) in sign_user_signed_action. The comment stated that signatureChainId can be any chain, but the code forced a testnet chain ID, causing potential wallet UX confusion for mainnet users.

Changes

• In hyperliquid/utils/signing.py: default signatureChainId to 0xa4b1 (Arbitrum One) for mainnet, 0x66eee for testnet, only if not already set by the caller.
• In hyperliquid/exchange.py: compute signatureChainId dynamically based on is_mainnet in the multi_sig method.
• The fix preserves backward compatibility: existing callers that set signatureChainId in the action dict will have it respected.

Testing

• Existing unit tests pass.
• Manual verification shows correct chain IDs for mainnet/testnet defaults and custom chain IDs are preserved.

Fixes #254