Fix playlist order (#13730)

* Fix playlist order move

* Remove extra space

* Added more test cases

* Change namespace to file-scoped

tests/Jellyfin.Server.Implementations.Tests/Playlists/PlaylistManagerTests.cs

@@ -0,0 +1,40 @@
+using Emby.Server.Implementations.Playlists;
+using Xunit;
+
+namespace Jellyfin.Server.Implementations.Tests.Playlists;
+
+public class PlaylistManagerTests
+{
+    [Fact]
+    public void DetermineAdjustedIndexMoveToFirstPositionNoPriorInAllList()
+    {
+        var priorIndexAllChildren = 0;
+        var newIndex = 0;
+
+        var adjustedIndex = PlaylistManager.DetermineAdjustedIndex(priorIndexAllChildren, newIndex);
+
+        Assert.Equal(0, adjustedIndex);
+    }
+
+    [Fact]
+    public void DetermineAdjustedIndexPriorInMiddleOfAllList()
+    {
+        var priorIndexAllChildren = 2;
+        var newIndex = 0;
+
+        var adjustedIndex = PlaylistManager.DetermineAdjustedIndex(priorIndexAllChildren, newIndex);
+
+        Assert.Equal(1, adjustedIndex);
+    }
+
+    [Fact]
+    public void DetermineAdjustedIndexMoveMiddleOfPlaylist()
+    {
+        var priorIndexAllChildren = 2;
+        var newIndex = 1;
+
+        var adjustedIndex = PlaylistManager.DetermineAdjustedIndex(priorIndexAllChildren, newIndex);
+
+        Assert.Equal(3, adjustedIndex);
+    }
+}