DeliveryApi: Ensure the ContentType property is serialized first

This is needed so it can be used as a discriminator field by System.Text.Json
Changed both ApiElement and IApiElement to ensure not only ApiContentResponse,
but also ApiBlockItem, which uses the interface directly, are adjusted.

src/Umbraco.Core/Models/DeliveryApi/ApiElement.cs

@@ -1,3 +1,5 @@
+using System.Text.Json.Serialization;
+
 namespace Umbraco.Cms.Core.Models.DeliveryApi;
 
 public class ApiElement : IApiElement
@@ -11,6 +13,9 @@ public class ApiElement : IApiElement
 
     public Guid Id { get; }
 
+    // Ensure the ContentType property is serialized first
+    // This is needed so it can be used as a discriminator field by System.Text.Json
+    [JsonPropertyOrder(-100)]
     public string ContentType { get; }
 
     public IDictionary<string, object?> Properties { get; }

src/Umbraco.Core/Models/DeliveryApi/IApiElement.cs

@@ -1,9 +1,14 @@
+using System.Text.Json.Serialization;
+
 namespace Umbraco.Cms.Core.Models.DeliveryApi;
 
 public interface IApiElement
 {
     Guid Id { get; }
 
+    // Ensure the ContentType property is serialized first
+    // This is needed so it can be used as a discriminator field by System.Text.Json
+    [JsonPropertyOrder(-100)]
     string ContentType { get; }
 
     IDictionary<string, object?> Properties { get; }