fix: Do not allow unregistered classes in JSON RPC interface

yarn-project/foundation/src/json-rpc/convert.ts

@@ -75,7 +75,10 @@ export function convertFromJsonObj(cc: ClassConverter, obj: any): any {
     }
     return newObj;
   }
-
+  // Throw if this is a non-primitive class that was not registered
+  if (typeof obj === 'object' && obj.constructor !== Object) {
+    throw new Error(`Object ${obj.constructor.name} not registered for deserialisation`);
+  }
   // Leave alone, assume JSON primitive
   return obj;
 }
@@ -118,7 +121,10 @@ export function convertToJsonObj(cc: ClassConverter, obj: any): any {
     }
     return newObj;
   }
-
+  // Throw if this is a non-primitive class that was not registered
+  if (typeof obj === 'object' && obj.constructor !== Object) {
+    throw new Error(`Object ${obj.constructor.name} not registered for serialisation`);
+  }
   // Leave alone, assume JSON primitive
   return obj;
 }