fix(errors): preserve original provider cause in getContractError

[jasonandjay]

Fixes #4387

Summary

When a custom provider throws an execution error (e.g. { code: 3, data, name: 'CustomProviderExecutionError' }), getContractError currently wraps it into ContractFunctionRevertedError but can lose the original nested cause.

This PR preserves that cause chain:

• getContractError now passes the discovered low-level error as cause into ContractFunctionRevertedError.
• ContractFunctionRevertedError now accepts an optional cause parameter and keeps it unless replaced by ABI decode failures.
• Added a regression test asserting error.cause.cause?.name === 'CustomProviderExecutionError'.

Notes

I couldn't fully run local repo checks in this environment because viem currently requires Node >= 24.5 and this runner is Node 22. CI should validate on the project's standard toolchain.

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[jxom]

already a pr for this #4388