fix(physics): validate retail cell containment roots

tests/AcDream.App.Tests/Input/PlayerMovementPlacementTransactionTests.cs

@@ -57,6 +57,13 @@ public sealed class PlayerMovementPlacementTransactionTests
                     Vertices = new Dictionary<ushort, SWVertex>(),
                 },
                 Polygons = new Dictionary<ushort, Polygon>(),
+                CellBSP = new CellBSPTree
+                {
+                    Root = new CellBSPNode
+                    {
+                        Type = DatReaderWriter.Enums.BSPNodeType.Leaf,
+                    },
+                },
             };
             var envCell = new DatEnvCell
             {