fix: Fix JSONRPCVoidResponseSerializer constructor (#193)

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Author: fjuma

spec/src/main/java/io/a2a/spec/JSONRPCVoidResponseSerializer.java

@@ -5,17 +5,15 @@
 import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.databind.SerializerProvider;
 import com.fasterxml.jackson.databind.ser.std.StdSerializer;
+import com.fasterxml.jackson.databind.type.TypeFactory;
 
 public class JSONRPCVoidResponseSerializer extends StdSerializer<JSONRPCResponse<Void>> {
 
     private static final JSONRPCErrorSerializer JSON_RPC_ERROR_SERIALIZER = new JSONRPCErrorSerializer();
 
     public JSONRPCVoidResponseSerializer() {
-        this(null);
-    }
-
-    public JSONRPCVoidResponseSerializer(Class<JSONRPCResponse<Void>> vc) {
-        super(vc);
+        super(TypeFactory.defaultInstance().constructParametricType(JSONRPCResponse.class,
+                Void.class));
     }
 
     @Override