Fixes #5442. Windows VT input can hang during shutdown when ReadFile is blocked

[BDisp]

Fixes

• Fixes Windows VT input can hang during shutdown when ReadFile is blocked #5442

Proposed Changes/Todos

• Add CancelIoEx API

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[Copilot]

Pull request overview

Fixes a Windows shutdown hang where the ANSI driver's input thread could be stuck inside synchronous ReadFile. Adds a CancelIoEx-based wake helper invoked during AnsiOutput.Dispose, and treats ERROR_OPERATION_ABORTED from ReadFile as normal cancellation rather than a warning.

Changes:

• Add CancelIoEx P/Invoke and a static WakePendingRead() helper on WindowsVTInputHelper.
• Silently treat ERROR_OPERATION_ABORTED (995) from ReadFile as a benign cancellation.
• Call WakePendingRead() from AnsiOutput.Dispose when running on the WindowsVT platform.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Terminal.Gui/Drivers/WindowsHelpers/WindowsVTInputHelper.cs	Adds CancelIoEx interop, error constants, WakePendingRead() helper, and handles ERROR_OPERATION_ABORTED in TryRead.
Terminal.Gui/Drivers/AnsiDriver/AnsiOutput.cs	Invokes WindowsVTInputHelper.WakePendingRead() during dispose on the WindowsVT platform to unblock pending reads.

[tig]

lgtm