Fix edge cases around .vhd support

[benhillis]

While experimenting with the new --fixed-vhd support I realized we do not correctly handle all cases where a distro might be stored as a .vhd. This resolves those issues and clarifies usage that we support both .vhd and .vhdx.

[benhillis]

Considering adding some tests, need to convert some vhdx to vhd.

[OneBlue]

Change looks good. Regarding test coverage, looks like https://learn.microsoft.com/en-us/powershell/module/hyper-v/convert-vhd?view=windowsserver2025-ps can convert from vhdx to vhd

[benhillis]

Change looks good. Regarding test coverage, looks like https://learn.microsoft.com/en-us/powershell/module/hyper-v/convert-vhd?view=windowsserver2025-ps can convert from vhdx to vhd

Done.

[Copilot]

Pull Request Overview

This PR improves VHD (Virtual Hard Disk) support in WSL by fixing edge cases and clarifying that both .vhd and .vhdx file formats are supported. The changes address issues discovered while testing the --fixed-vhd feature.

Key Changes

• Updated error messages and documentation to reflect support for both .vhd and .vhdx file extensions
• Added validation logic to prevent export/import of VHD files to/from stdin/stdout
• Implemented dynamic VHD filename handling in move and registration operations to preserve file extensions

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/windows/common/wslutil.h	Added new IsVhdFile() helper function declaration
src/windows/common/wslutil.cpp	Implemented IsVhdFile() to check for both .vhd and .vhdx extensions
src/windows/common/WslClient.cpp	Removed client-side file extension validation (moved to service)
src/windows/service/exe/LxssUserSession.cpp	Added service-side validation for VHD export/import operations and dynamic filename handling
src/windows/service/exe/WslCoreFilesystem.cpp	Simplified assertion to use new IsVhdFile() helper
localization/strings/en-US/Resources.resw	Updated error messages and help text to clarify .vhd/.vhdx support
test/windows/UnitTests.cpp	Updated test assertions and added comprehensive ExportImportVhd test
test/windows/SimpleTests.cpp	Updated test assertions for consistency