Fix test broken by autofix and use json variable in deserialize call

The Copilot Autofix commit removed the DeserializeWrapper call and
Assert.IsType assertion from NonPublicProperty_JsonInclude_WorksAsExpected
while leaving references to the now-undeclared 'result' local, breaking
compilation. Restore the deserialize call (using the existing 'json'
variable per review feedback) and the type assertion.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: eiriktsarpalis

src/libraries/System.Text.Json/tests/Common/PropertyVisibilityTests.NonPublicAccessors.cs

@@ -365,6 +365,8 @@ public virtual async Task NonPublicProperty_JsonInclude_WorksAsExpected(Type typ
             string json = """{"MyString":"value"}""";
             MemberInfo memberInfo = type.GetMember("MyString", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance)[0];
 
+            object result = await Serializer.DeserializeWrapper(json, type);
+            Assert.IsType(type, result);
             Assert.Equal("value", memberInfo is PropertyInfo p ? p.GetValue(result) : ((FieldInfo)memberInfo).GetValue(result));
 
             string actualJson = await Serializer.SerializeWrapper(result, type);