fix: ReceiptsRoot 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

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