test: update signing test username

backend/mpc-system/test_signing.go

@@ -68,7 +68,7 @@ func main() {
 	// Create signing session via account-service
 	// This account has threshold_t=1, so should only need 2 parties (t+1=2)
 	signingData := map[string]interface{}{
-		"username":     "wallet-f13135f7",
+		"username":     "wallet-fcac966d",
 		"message_hash": messageHashHex,
 	}