Client should receive event with error code and message when sending the sign event with the same tx_id

[anhthii]

When signing through mpcClient.SignTransaction, if types.SignTxMessage.Tx has been used, mpcClient.OnSignResult cannot receive the result, resulting in the inability to continue processing.

The backend reports the following information
2025-07-26 14:20:10.000 INF Duplicate signing request detected txID=55e1ca81-8f2b-4fc7-b522-eed44481d869 walletID=9664f6ce-89fc-4a4f-9767-da05a10bad22

Condition:Sending sign event with the same tx_id

Expectation: mpcClient.OnSignResult receives the corresponding error message

[Azzurriii]

@anhthii I’ve opened PR #63 to address this issue.
The fix ensures that onSignResult receives an error event when a duplicate signing request is detected.
Please take your time to review it.