refactor: Use typed errors for EthTrace::error instead of Option<String>

[akaladarshi]

Summary

The EthTrace struct in src/rpc/methods/eth/trace/types.rs currently stores its error
field as Option<String>:

pub struct EthTrace {
    // ...
    pub error: Option<String>,
}

This leads to unnecessary heap allocations, string comparisons, and lacks strong typing.

Completion Criteria

• Use Typed errors via thiserror

Additional Links & Resources

Original PR: #6751
Comment Thread: #6751 (comment)