fix: fixed removing current directory not throwing ioexception

[pw-sgr]

Closes #939

[Copilot]

Pull request overview

This PR aligns MockFileSystem behavior with Windows by preventing delete/move operations on the current working directory (or its parents) and throwing the same IOException patterns observed on the real file system (per #939).

Changes:

• Added Windows-specific tests asserting IOException when deleting the current directory (and nested current directory) via both Directory and DirectoryInfo.
• Added Windows-specific tests asserting IOException when moving the current directory (and when the current directory is nested under the moved directory) via both Directory and DirectoryInfo.
• Implemented Windows-only “current directory handle” validations in InMemoryStorage and introduced new exception factory helpers.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
Tests/Testably.Abstractions.Tests/FileSystem/DirectoryInfo/MoveToTests.cs	Adds Windows-only coverage for moving current directory scenarios.
Tests/Testably.Abstractions.Tests/FileSystem/DirectoryInfo/DeleteTests.cs	Adds Windows-only coverage for deleting current directory scenarios.
Tests/Testably.Abstractions.Tests/FileSystem/Directory/MoveTests.cs	Adds Windows-only coverage for moving current directory scenarios.
Tests/Testably.Abstractions.Tests/FileSystem/Directory/DeleteTests.cs	Adds Windows-only coverage for deleting current directory scenarios.
Source/Testably.Abstractions.Testing/Storage/InMemoryStorage.cs	Adds Windows-only validations that block delete/move when CWD is affected.
Source/Testably.Abstractions.Testing/Helpers/ExceptionFactory.cs	Adds new helper exception constructors used by the new validations.