fix: Fixes map iterators for Items (#1564)

### Summary

Modifying a ValueLinkList using one of the methods will bump the "version". This field is used by iterators (foreach loops) to determine if the link list was modified while iterating. The sector.Items (and in the future other lists), will no longer be safe to modify while iterating. The server will _CRASH_ if the ValueLinkList is modified.

Thanks to @stefanomerotta for help!


### Screenshots
<img width="588" alt="image" src="https://github.com/modernuo/ModernUO/assets/3953314/83ee0b6e-ff4f-4768-9e29-84456e04b1ec">

Projects/Server.Tests/Tests/Collections/ValueLinkListTests.cs

@@ -1,3 +1,4 @@
+using System;
 using Server.Collections;
 using Xunit;
 
@@ -32,20 +33,35 @@ public class ValueLinkListTests
 
         Assert.True(entity1.OnLinkList);
         Assert.Equal(1, linkList.Count);
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity1, linkList.Last);
+
+        Assert.Collection(linkList.ToArray(), item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Null(item.Previous);
+                Assert.Null(item.Next);
+            }
+        );
 
         var entity2 = new TestEntity(2);
-
         linkList.AddFirst(entity2);
 
         Assert.True(entity2.OnLinkList);
         Assert.Equal(2, linkList.Count);
-        Assert.Equal(entity1, linkList.Last);
-        Assert.Equal(entity2, entity1.Previous);
 
-        Assert.Equal(entity2, linkList.First);
-        Assert.Equal(entity1, entity2.Next);
+        Assert.Collection(linkList.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity2, item);
+                Assert.Null(item.Previous);
+                Assert.Equal(item.Next, entity1);
+            },
+            item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Equal(item.Previous, entity2);
+                Assert.Null(item.Next);
+            }
+        );
     }
 
     [Fact]
@@ -58,8 +74,14 @@ public class ValueLinkListTests
 
         Assert.True(entity1.OnLinkList);
         Assert.Equal(1, linkList.Count);
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity1, linkList.Last);
+
+        Assert.Collection(linkList.ToArray(), item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Null(item.Previous);
+                Assert.Null(item.Next);
+            }
+        );
 
         var entity2 = new TestEntity(2);
 
@@ -67,11 +89,21 @@ public class ValueLinkListTests
 
         Assert.True(entity2.OnLinkList);
         Assert.Equal(2, linkList.Count);
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity2, entity1.Next);
 
-        Assert.Equal(entity2, linkList.Last);
-        Assert.Equal(entity1, entity2.Previous);
+        Assert.Collection(linkList.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Null(item.Previous);
+                Assert.Equal(item.Next, entity2);
+            },
+            item =>
+            {
+                Assert.Equal(entity2, item);
+                Assert.Equal(item.Previous, entity1);
+                Assert.Null(item.Next);
+            }
+        );
     }
 
     [Fact]
@@ -87,20 +119,48 @@ public class ValueLinkListTests
         Assert.True(entity1.OnLinkList);
         Assert.True(entity2.OnLinkList);
         Assert.Equal(2, linkList.Count);
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity2, linkList.Last);
+
+        Assert.Collection(linkList.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Null(item.Previous);
+                Assert.Equal(item.Next, entity2);
+            },
+            item =>
+            {
+                Assert.Equal(entity2, item);
+                Assert.Equal(item.Previous, entity1);
+                Assert.Null(item.Next);
+            }
+        );
+
 
         var entity3 = new TestEntity(3);
         linkList.AddBefore(entity2, entity3);
         Assert.True(entity3.OnLinkList);
         Assert.Equal(3, linkList.Count);
 
-        Assert.Equal(entity1, entity3.Previous);
-        Assert.Equal(entity2, entity3.Next);
-
-        // First and Last should not have changed
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity2, linkList.Last);
+        Assert.Collection(linkList.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Null(item.Previous);
+                Assert.Equal(item.Next, entity3);
+            },
+            item =>
+            {
+                Assert.Equal(entity3, item);
+                Assert.Equal(item.Previous, entity1);
+                Assert.Equal(item.Next, entity2);
+            },
+            item =>
+            {
+                Assert.Equal(entity2, item);
+                Assert.Equal(item.Previous, entity3);
+                Assert.Null(item.Next);
+            }
+        );
     }
 
     [Fact]
@@ -113,23 +173,31 @@ public class ValueLinkListTests
         linkList.AddFirst(entity1);
         linkList.AddLast(entity2);
 
-        Assert.True(entity1.OnLinkList);
-        Assert.True(entity2.OnLinkList);
-        Assert.Equal(2, linkList.Count);
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity2, linkList.Last);
-
         var entity3 = new TestEntity(3);
         linkList.AddAfter(entity1, entity3);
         Assert.True(entity3.OnLinkList);
         Assert.Equal(3, linkList.Count);
 
-        Assert.Equal(entity1, entity3.Previous);
-        Assert.Equal(entity2, entity3.Next);
-
-        // First and Last should not have changed
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity2, linkList.Last);
+        Assert.Collection(linkList.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Null(item.Previous);
+                Assert.Equal(item.Next, entity3);
+            },
+            item =>
+            {
+                Assert.Equal(entity3, item);
+                Assert.Equal(item.Previous, entity1);
+                Assert.Equal(item.Next, entity2);
+            },
+            item =>
+            {
+                Assert.Equal(entity2, item);
+                Assert.Equal(item.Previous, entity3);
+                Assert.Null(item.Next);
+            }
+        );
     }
 
     [Fact]
@@ -157,12 +225,47 @@ public class ValueLinkListTests
         Assert.Equal(1, linkList.Count);
         Assert.Equal(5, linkList2.Count);
 
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity1, linkList.Last);
+        Assert.Collection(linkList.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Null(item.Previous);
+                Assert.Null(item.Next);
+            }
+        );
 
-        Assert.Equal(entity2, entity6.Next);
-        Assert.Equal(entity6, entity2.Previous);
-        Assert.Equal(entity4, linkList2.Last);
+        Assert.Collection(linkList2.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity5, item);
+                Assert.Null(item.Previous);
+                Assert.Equal(item.Next, entity6);
+            },
+            item =>
+            {
+                Assert.Equal(entity6, item);
+                Assert.Equal(item.Previous, entity5);
+                Assert.Equal(item.Next, entity2);
+            },
+            item =>
+            {
+                Assert.Equal(entity2, item);
+                Assert.Equal(item.Previous, entity6);
+                Assert.Equal(item.Next, entity3);
+            },
+            item =>
+            {
+                Assert.Equal(entity3, item);
+                Assert.Equal(item.Previous, entity2);
+                Assert.Equal(item.Next, entity4);
+            },
+            item =>
+            {
+                Assert.Equal(entity4, item);
+                Assert.Equal(item.Previous, entity3);
+                Assert.Null(item.Next);
+            }
+        );
     }
 
     [Fact]
@@ -184,10 +287,20 @@ public class ValueLinkListTests
         linkList.RemoveAllBefore(entity4);
 
         Assert.Equal(2, linkList.Count);
-        Assert.Equal(entity4, linkList.First);
-        Assert.Equal(entity5, linkList.Last);
-        Assert.Null(entity4.Previous);
-        Assert.Null(entity5.Next);
+        Assert.Collection(linkList.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity4, item);
+                Assert.Null(item.Previous);
+                Assert.Equal(item.Next, entity5);
+            },
+            item =>
+            {
+                Assert.Equal(entity5, item);
+                Assert.Equal(item.Previous, entity4);
+                Assert.Null(item.Next);
+            }
+        );
     }
 
     [Fact]
@@ -209,9 +322,104 @@ public class ValueLinkListTests
         linkList.RemoveAllAfter(entity2);
 
         Assert.Equal(2, linkList.Count);
-        Assert.Equal(entity1, linkList.First);
-        Assert.Equal(entity2, linkList.Last);
-        Assert.Null(entity1.Previous);
-        Assert.Null(entity2.Next);
+        Assert.Collection(linkList.ToArray(),
+            item =>
+            {
+                Assert.Equal(entity1, item);
+                Assert.Null(item.Previous);
+                Assert.Equal(item.Next, entity2);
+            },
+            item =>
+            {
+                Assert.Equal(entity2, item);
+                Assert.Equal(item.Previous, entity1);
+                Assert.Null(item.Next);
+            }
+        );
+    }
+
+    [Fact]
+    public void TestVersionIncrements()
+    {
+        var linkList = new ValueLinkList<TestEntity>();
+
+        var entity1 = new TestEntity(1);
+        var entity2 = new TestEntity(2);
+        var entity3 = new TestEntity(3);
+        var entity4 = new TestEntity(4);
+
+        var version = 0;
+        Assert.Equal(version, linkList.Version);
+
+        linkList.AddFirst(entity1); // 1
+        Assert.Equal(++version, linkList.Version);
+
+        linkList.AddLast(entity2); // 1, 2
+        Assert.Equal(++version, linkList.Version);
+
+        linkList.AddBefore(entity2, entity3); // 1, 3, 2
+        Assert.Equal(++version, linkList.Version);
+
+        linkList.AddAfter(entity3, entity4); // 1, 3, 4, 2
+        Assert.Equal(++version, linkList.Version);
+
+        linkList.Remove(entity1); // 3, 4, 2
+        Assert.Equal(++version, linkList.Version);
+
+        linkList.RemoveAllAfter(entity4); // 3, 4
+        Assert.Equal(++version, linkList.Version);
+
+        linkList.RemoveAllBefore(entity4); // 4
+        Assert.Equal(++version, linkList.Version);
+
+        linkList.RemoveAll(); // None
+        Assert.Equal(++version, linkList.Version);
+    }
+
+    [Fact]
+    public void TestThrowsIfModifiedWhileIterating()
+    {
+        Assert.Throws<InvalidOperationException>(
+            () =>
+            {
+                var linkList = new ValueLinkList<TestEntity>();
+
+                var entity1 = new TestEntity(1);
+                var entity2 = new TestEntity(2);
+
+                linkList.AddFirst(entity1);
+                linkList.AddLast(entity2);
+
+                foreach (var item in linkList)
+                {
+                    linkList.Remove(item);
+                }
+            }
+        );
+    }
+
+    [Fact]
+    public void TestThrowsIfModifiedWhileIteratingNested()
+    {
+        Assert.Throws<InvalidOperationException>(
+            () =>
+            {
+                var linkList = new ValueLinkList<TestEntity>();
+
+                var entity1 = new TestEntity(1);
+                var entity2 = new TestEntity(2);
+
+                linkList.AddFirst(entity1);
+                linkList.AddLast(entity2);
+
+                foreach (var item in linkList)
+                {
+                    foreach (var nestedItem in linkList)
+                    {
+                        linkList.Remove(nestedItem);
+                    }
+                }
+            }
+        );
     }
 }