fix(traces): decode return_data for Identity precompile

[fifalodm]

Motivation

The decode function for precompiles had a TODO to decode return_data.
For the Identity precompile (0x04), this is trivial since the output is exactly the same as the input, making it safe and easy to implement.

Solution

• Implemented decoding of return_data for the Identity precompile.
• Used hex::encode_prefixed(&trace.output) to match existing hex formatting in the project.
• Kept label "PRECOMPILES" and followed the existing style of the decoder.

    let return_data =
        if trace.address == IDENTITY { Some(hex::encode_prefixed(&trace.output)) } else { None };

PR Checklist

• No new tests needed (change is trivial and affects debug output only)
• No new documentation needed
• No breaking changes