fix SafeFileHandle.CanSeek performance regression

[adamsitnik]

fixes #125660

[adamsitnik]

@EgorBot -windows_intel

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Benchmarks).Assembly).Run(args);

[MemoryDiagnoser]
public class Benchmarks
{
    private string? _filePath;

    [GlobalSetup]
    public void Setup()
    {
        _filePath = Path.GetTempFileName();
        File.WriteAllBytes(_filePath, new byte[1_000]);
    }

    [GlobalCleanup]
    public void Cleanup() => File.Delete(_filePath!);

    [Benchmark]
    public byte[] ReadAllBytes() => File.ReadAllBytes(_filePath!);

    [Benchmark]
    public Task ReadAllBytesAsync() => File.ReadAllBytesAsync(_filePath!);

    [Benchmark]
    public bool OpenAndCanSeek()
    {
        using FileStream handle = File.OpenRead(_filePath!);
        return handle.CanSeek;
    }
}

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[Copilot]

Pull request overview

This PR targets a Windows performance regression in SafeFileHandle.CanSeek / SafeFileHandle.Type usage by avoiding an extra handle-type probe for common disk-file handles.

Changes:

• Adds a GetFileTypeCore fast-path for FILE_TYPE_DISK handles when Path is available, returning RegularFile without calling GetDiskBasedType().

[Copilot]

The new fast-path assumes Path is not null implies the handle can't be a directory or a symlink, but SafeFileHandle.Open passes through FileOptions and FileStreamHelpers.ValidateArguments explicitly allows (FileOptions)0x02000000 (FILE_FLAG_BACKUP_SEMANTICS). With that flag, CreateFile can successfully open directory handles while still setting _path, so this switch arm would misclassify such handles as RegularFile (making Type/CanSeek incorrect). Consider narrowing the fast-path to cases where FILE_FLAG_BACKUP_SEMANTICS is not set (or otherwise proving directories can’t be opened) and adjust the comment accordingly.

Suggested change

	// which does not allow opening directories or symbolic links (it resolves them).
	// That is why we can skip the extra check performed by GetDiskBasedType.
	Interop.Kernel32.FileTypes.FILE_TYPE_DISK when Path is not null => FileHandleType.RegularFile,
	// which does not allow opening directories or symbolic links (it resolves them) unless
	// FILE_FLAG_BACKUP_SEMANTICS (0x02000000) is specified in FileOptions to allow opening directories.
	// When backup semantics are not used, we can skip the extra check performed by GetDiskBasedType.
	Interop.Kernel32.FileTypes.FILE_TYPE_DISK when Path is not null && (GetFileOptions() & (FileOptions)0x02000000) == 0 => FileHandleType.RegularFile,