fix: Improve typing for EIP712 transactions

[DeVikingMark]

This PR addresses the TODO comment in signTransaction.ts by introducing a proper
typing for EIP712 transactions instead of using as any type assertions.

Changes:

• Added a TransactionWithType helper type that extends ZksyncTransactionRequest
• Replaced inline "as any" casts with the proper type
• Made the code more type-safe while maintaining compatibility with the viem library

These changes improve code quality and type safety without changing the actual behavior.

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PR-Codex overview

This PR introduces a new type helper for handling EIP-712 transactions in the signTransaction functionality, ensuring proper type assignment for transactions and enhancing type safety.

Detailed summary

• Added a new type TransactionWithType for EIP-712 transactions.
• Updated signEip712TransactionInternal to properly assign the transaction type.
• Modified type casting for transaction to utilize TransactionWithType.

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