V16: Siblings endpoints (#19657)

* PoC implementation

* Move to controller base

* Implement solution that seems worse, but works better

* Don't require parent key in repository method

* Fix typos

* Add siblings for data type, media type and media

* Add endpoint for template

* Add DocumentType and DocumentBlueprint controllers

* Fix naming

* Fix case if siblings are under root

* Take item ordering into account

not all entities are ordered by sort order

* Add default implementation

* Fix parentkey

* Add tests

* Format optimizations for split view

* Add test covered requirement to description

* Cover positive case and make test case output more readable

* reduce allocations

* Clarify test

---------

Co-authored-by: Migaroez <geusens@gmail.com>

tests/Umbraco.Tests.UnitTests/Umbraco.Core/Services/EntityServiceTests.cs

@@ -0,0 +1,38 @@
+using Microsoft.AspNetCore.Components;
+using Microsoft.Extensions.Logging;
+using Moq;
+using NUnit.Framework;
+using Umbraco.Cms.Core.Events;
+using Umbraco.Cms.Core.Models;
+using Umbraco.Cms.Core.Persistence.Repositories;
+using Umbraco.Cms.Core.Scoping;
+using Umbraco.Cms.Core.Services;
+
+namespace Umbraco.Cms.Tests.UnitTests.Umbraco.Core.Services;
+
+[TestFixture]
+public class EntityServiceTests
+{
+    [TestCase(1, 1, false, TestName = "Siblings_Index_Validation_Valid")]
+    [TestCase(-1, 1, true, TestName = "Siblings_Index_Validation_InvalidBefore")]
+    [TestCase(1, -1, true, TestName = "Siblings_Index_Validation_InvalidAfter")]
+    [TestCase(-1, -1, true, TestName = "Siblings_Index_Validation_InvalidBeforeAndAfter")]
+    public void Siblings_Index_Validation(int before, int after, bool shouldThrow)
+    {
+        var sut = CreateEntityService();
+
+        if (shouldThrow)
+        {
+            Assert.Throws<ArgumentOutOfRangeException>(() => sut.GetSiblings(Guid.NewGuid(), UmbracoObjectTypes.Document, before, after));
+        }
+    }
+
+
+    private EntityService CreateEntityService() =>
+        new(
+            Mock.Of<ICoreScopeProvider>(),
+            Mock.Of<ILoggerFactory>(),
+            Mock.Of<IEventMessagesFactory>(),
+            Mock.Of<IIdKeyMap>(),
+            Mock.Of<IEntityRepository>());
+}