MockFileSystem incorrectly allows shared access to same file

[maltebp]

Describe the bug
According to C#'s filesystem API, if you open a file with FileShare.None then it should not be possible to create other file streams to that file. If you do this using the "real" file system on Windows, it will throw an IOException with the message: "The process cannot access the file '-local file path-' because it is being used by another process.". But using the MockFileSystem this does not produce any error, which seems wrong.

To Reproduce
Run this code:

 MockFileSystem mockFileSystem = new();
 mockFileSystem.FileStream.New("SomeFile.txt", FileMode.Create, FileAccess.ReadWrite, FileShare.None); // Should cause open to fail
 // mockFileSystem.File.Create("SomeFile.txt"); // Does not fail either

 mockFileSystem.FileStream.New("SomeFile.txt", FileMode.Open, FileAccess.Read);


 FileSystem realFileSystem = new();
 realFileSystem.FileStream.New("SomeFile.txt", FileMode.Create, FileAccess.ReadWrite, FileShare.None); // Causes open to fail
 // realFileSystem.File.Create("SomeFile.txt"); Also fails

 realFileSystem.FileStream.New("SomeFile.txt", FileMode.Open, FileAccess.Read);

Expected behavior
It should fail when creating the "open" stream on the MockFileSystem, but it does not.

Additional context

• TestableIO version: 22.0.16
• .NET: net8.0-windows
• Windows version: Microsoft Windows 11 Enterprise (10.0.22631 Build 22631)