Fix Windows COPY --parents with skip protected system files

[rzlink]

Fixes copying from Windows container mount roots by detecting and skipping protected system files ('System Volume Information' and 'WcSandboxState') that cause 'Access is denied' errors.

Changes:

• Added platformCopy wrapper for Windows copy operations
• Implemented manual directory enumeration to skip protected files
• Enabled test case testCopyRelativeParents and testCopyParentsMissingDirectory on Windows

The fix is Windows-specific and only activates when copying from mount roots where protected files exist. All other copy operations use the standard path.

Issue: Resolves #6635 [v0.26] WCOW: Fix COPY --parents tests

[tonistiigi]

Could this be simplified when integrating directly to copy.Copy where it already calls RootPath and ReadDir ?

[rzlink]

Could this be simplified when integrating directly to copy.Copy where it already calls RootPath and ReadDir ?

Yes, it will simplify to:

func platformCopy(ctx context.Context, srcRoot string, src string, destRoot string, dest string, opt ...copy.Opt) error {
	// Add Windows protected files to exclude patterns
	opt = append(opt,
		copy.WithExcludePattern("System Volume Information"),
		copy.WithExcludePattern("WcSandboxState"),
	)
	return copy.Copy(ctx, srcRoot, src, destRoot, dest, opt...)
}