test: update test_signing.go to use current wallet username

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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-76e34337",
+		"username":     "wallet-fb51d689",
 		"message_hash": messageHashHex,
 	}