fix legacy structLog JSON encoding

[GheisMohammadi]

This PR makes error field truly optional in tracer logger. StructLogRes.Error was a string. Even when there was no error, JSON had "error": "". After changes, It’s a *string with omitempty, and we only set it when there is a real error message. Many tracers/INDEXERS/specs expect the field to be absent when no error occurs, not present with "". This fixes schema/decoder issues and false “error” detections.
Current memory was chunked into 32‑byte slices and encoded with fmt.Sprintf("%x", slice). A partial word (e.g. 2 bytes) gave "0xaabb" instead of a 32‑byte (64‑hex‑char) word. After changes each memory chunk is:

• Cut into up to 32 bytes,
• Zero‑padded to 32 bytes,
• Hex‑encoded via hexutil.Bytes(word).String() → 0x + 64 hex chars.
  Tools that assume 32‑byte, zero‑padded, 0x‑prefixed words (as in Geth) no longer break or misinterpret short words at the end of memory.

Also, the PR helps the storage and return data hex encoding is now consistent. In current code:
Storage keys/values were fmt.Sprintf("%x", hash) → no 0x prefix, inconsistent with most Ethereum JSON APIs.
ExecutionResult.ReturnValue was a plain hex string and manually blanked out on failure.
But the PR changes it to:

• Storage uses hash.Hex() for both key and value → consistent 0x‑prefixed hex.
• ReturnValue is hexutil.Bytes, and we zero the underlying bytes on hard failure.

So, now debuggers and explorers that expect standard 0x‑prefixed hex (like the Execution APIs / Geth) can work with your traces without custom handling.

The PR fixes debug_traceTransaction style output’s JSON shape and hex formats now match what existing Ethereum tooling already knows how to parse.

Note: the same changes on Ethereum can be found on PR#34093

[mur-me]

@mur-me - check the failed rpc tests, fix should be simple, see the golang tests from this PR