fix: receipts root encoding

[dudong2]

Description

tl;dr

Currently, EmitReceiptsRootEvent emits receiptsRoot as string(receiptsRoot.Bytes()), which may contain non-UTF-8 characters and result in unreadable or invalid output.
This PR changes the emission to use receiptsRoot.Hex() instead, which is UTF-8 safe and human-readable.

Problem

• string(receiptsRoot.Bytes()) can include invalid UTF-8 bytes (e.g. 0x00, 0x80–0xFF)
• This leads to:
  • Escaped or malformed data in RPC responses
  • Inconsistent hash reconstruction in clients

FIx

sdk.NewAttribute(types.AttributeKeyEthereumReceiptsRoot, receiptsRoot.Hex())

Impact

• Safer and cleaner receiptsRoot in events
• No behavioral change — only improves compatibility with RPC consumers

Closes: #XXXX

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• tackled an existing issue or discussed with a team member
• left instructions on how to review the changes
• targeted the main branch

Reviewers Checklist

All items are required.
Please add a note if the item is not applicable
and please add your handle next to the items reviewed
if you only reviewed selected items.

I have...

• added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• confirmed all author checklist items have been addressed
• confirmed that this PR does not change production code
• reviewed content
• tested instructions (if applicable)
• confirmed all CI checks have passed