opStack: use L1 chain for viem getTimeToFinalize

[VolodymyrBg]

• getTimeToFinalize executes on L1 and expects the L1 chain object; previously it used VIEM_OP_STACK_CHAINS[hubChainId], which maps L2 chains and is incorrect for an L1 id.
• Using an L2 mapping keyed by an L1 id can resolve to undefined and/or query the wrong chain, producing incorrect challenge-period timings.
• This aligns with surrounding viem calls that already pass L1 (getWithdrawalStatus, getL2Output) and keeps the L2 chain only for buildProveWithdrawal.