Describe the bug

Bug #15718 reveals issues that arise with the upgrade to System.Text.Json when using dynamic typing. Previously, Newtonsoft.Json handled many automatic conversions for JSON values, facilitating the use of dynamic typing with various target types. For example, in

With the new dynamic support added by @jtkech, these conversions are absent. All values are returned as parsed by System.Text.Json and converted to an object, which only returns types defined by JsonValueKind. This behavior is implemented here: JsonDynamicValue.cs#L28.

This change breaks all dynamic usages with specific target types.

To Reproduce

See #15718 for steps how to reproduce this error, but the underlaying problem is not limited to GraphQL.

As another example, the following unit test passes with Newtonsoft.Jsonbut fails now with STJ:

[Fact]
public void ShouldDeserializeDateTimeFields()
{
    var contentItem = new ContentItem();
    contentItem.GetOrCreate<MyPart>();
    contentItem.Alter<MyPart>(x => x.Text = "test");
    contentItem.Alter<MyPart>(x =>
    {
        x.GetOrCreate<MyDateTimeField>("myField");
        x.Alter<MyDateTimeField>("myField", f => f.Value = new DateTime(2024, 1, 1, 10, 42, 0));
    });

    var json = JsonConvert.SerializeObject(contentItem);

    var contentItem2 = JConvert.DeserializeObject<ContentItem>(json);

    Assert.NotNull(contentItem2.Content.MyPart);
    Assert.NotNull(contentItem2.Content.MyPart.myField);
    Assert.Equal(new DateTime(2024, 1, 1, 10, 42, 0), (DateTime?)contentItem2.Content.MyPart.myField.Value);
}

public class MyDateTimeField : ContentField
{
    public DateTime? Value { get; set; }
}

Expected behavior

To maintain compatibility and not disrupt existing functionality that uses dynamic types, conversions similar to those provided by Newtonsoft.Json should still be possible.