Fix deterministic MountVolume test failures on ARM64 Helix machines (#126660)

> [!NOTE]
> This PR was created with Copilot assistance.

## Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes #125295, fixes #125624, fixes #126627

### Problem

`Directory_Delete_MountVolume.RunTest` and
`Directory_ReparsePoints_MountVolume.runTest` fail deterministically
(~100% of the time, ~750ms duration) on the `Windows.11.Arm64.Open`
Helix machine pool. This is **not timing-related** and was not addressed
by the delay/polling fixes in #125914 or the Unmount resilience fix in
#125625 (those PRs fixed real timing issues -- pre-fix failures on other
configurations have since expired from AzDO retention, so we can't
verify directly, but there is no evidence they were ineffective for
their intended purpose).

**Root cause**: The ARM64 Helix machines have an E:\ drive (likely an
Azure resource/temp disk) that passes all `DriveInfo` checks --
`DriveType=Fixed`, `DriveFormat=NTFS`, `IsReady=True` -- but
`GetVolumeNameForVolumeMountPoint` fails with `ERROR_INVALID_PARAMETER`
(87). The drive has no volume GUID and doesn't support volume mount
point operations. `IOServices.GetNtfsDriveOtherThanCurrent()` returns
this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at
all). On those machines, the cross-drive scenarios already skip
gracefully and only same-drive scenarios 3.x run.

### Evidence

Analyzed Helix console logs from 5 post-fix builds (all
`arm64-NativeAOT-Win11`, same C:\ volume GUID). Every failure shows the
identical pattern:
- Scenario 1: `GetVolumeNameForVolumeMountPoint("E:\")` -> error 87
- Scenario 2: `SetVolumeMountPoint` onto E:\ succeeds but path traversal
through the mount point fails with `DirectoryNotFoundException`
- Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating
`SUBST E:` which exhibits identical error 87 behavior.

### Changes

1. **`IOServices.GetNtfsDriveOtherThan()`**: After the existing
Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via
`GetVolumeNameForVolumeMountPoint`. Drives without one (SUBST drives,
Azure resource disks) are skipped.

2. **`DumpDriveInformation` diagnostic test**: New Helix-only test
(following the `DescriptionNameTests.DumpRuntimeInformationToConsole`
pattern) that dumps all drives with their volume GUIDs to the console
log. Makes future drive-related CI issues immediately diagnosable from
the same Helix work item log.

3. **`GetVolumeNameForVolumeMountPoint` P/Invoke in DllImports.cs**:
Uses `char[]` (not `StringBuilder`) because this file uses
`LibraryImport` which does not support `StringBuilder`.

### Local validation

| Scenario | Before fix | After fix |
|---|---|---|
| SUBST E: (no volume GUID) | Error 87 / DirectoryNotFoundException |
Pass (SUBST filtered, scenarios 3.x run) |
| Real NTFS E: | Pass (all scenarios) | Pass (all scenarios) |
| Single-drive machine | Scenarios 1/2 skip, 3.x pass | Same -- no
change |

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: danmoseley

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs

@@ -0,0 +1,82 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.IO;
+using System.Runtime.InteropServices;
+using Microsoft.DotNet.XUnitExtensions;
+using Xunit;
+
+namespace System.IO.Tests
+{
+    public class DumpDriveInformation
+    {
+        // When running both inner and outer loop together, dump only once
+        private static bool s_dumped = false;
+
+        [Fact]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.Wasi, "Not applicable")]
+        public void DumpDriveInformationToConsole()
+        {
+            if (s_dumped || !PlatformDetection.IsInHelix)
+                return;
+
+            s_dumped = true;
+
+            // Not really a test, but useful to dump drive/volume information to the test log
+            // to help debug environmental issues with mount volume tests in CI.
+            // Follows the pattern of DescriptionNameTests.DumpRuntimeInformationToConsole.
+
+            Console.WriteLine("### DRIVE INFORMATION");
+            Console.WriteLine($"###   Machine: {Environment.MachineName}");
+            Console.WriteLine($"###   OS: {RuntimeInformation.OSDescription} ({RuntimeInformation.OSArchitecture})");
+            Console.WriteLine($"###   Current directory: {Directory.GetCurrentDirectory()}");
+
+            foreach (DriveInfo drive in DriveInfo.GetDrives())
+            {
+                try
+                {
+                    string info = $"###   Drive {drive.Name}: Type={drive.DriveType}";
+                    if (drive.IsReady)
+                    {
+                        info += $" Format={drive.DriveFormat} Size={drive.TotalSize / (1024 * 1024)}MB";
+
+                        if (OperatingSystem.IsWindows())
+                        {
+                            char[] volName = new char[260];
+                            bool hasGuid = DllImports.GetVolumeNameForVolumeMountPoint(drive.Name, volName, volName.Length);
+                            info += hasGuid
+                                ? $" VolumeGUID={new string(volName).TrimEnd('\0')}"
+                                : $" VolumeGUID=NONE({Marshal.GetLastPInvokeErrorMessage()})";
+                        }
+                    }
+                    else
+                    {
+                        info += " NotReady";
+                    }
+                    Console.WriteLine(info);
+                }
+                catch (Exception ex)
+                {
+                    Console.WriteLine($"###   Drive {drive.Name}: error probing: {ex.GetType().Name}: {ex.Message}");
+                }
+            }
+
+            if (OperatingSystem.IsWindows())
+            {
+                string otherNtfs = IOServices.GetNtfsDriveOtherThanCurrent();
+                Console.WriteLine($"###   GetNtfsDriveOtherThanCurrent() = {otherNtfs ?? "(null)"}");
+            }
+        }
+
+        [Fact]
+        [OuterLoop]
+        [SkipOnPlatform(TestPlatforms.Browser | TestPlatforms.Wasi, "Not applicable")]
+        public void DumpDriveInformationToConsoleOuter()
+        {
+            // Outer loop runs don't run inner loop tests.
+            // But we want to log this data for any Helix run.
+            DumpDriveInformationToConsole();
+        }
+    }
+}

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Runtime.InteropServices;
-using System.Text;
 
 internal static partial class DllImports
 {
@@ -16,4 +15,8 @@ internal static partial class DllImports
 
     [LibraryImport("kernel32.dll", EntryPoint = "GetDriveTypeW", StringMarshalling = StringMarshalling.Utf16, SetLastError = true)]
     internal static partial int GetDriveType(string drive);
+
+    [LibraryImport("kernel32.dll", EntryPoint = "GetVolumeNameForVolumeMountPointW", StringMarshalling = StringMarshalling.Utf16, SetLastError = true)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    internal static partial bool GetVolumeNameForVolumeMountPoint(string volumeMountPoint, [Out] char[] volumeName, int bufferLength);
 }

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs

@@ -64,13 +64,27 @@ public static string GetNtfsDriveOtherThan(string drive)
             if (!IsReady(otherDrive))
                 continue;
 
-            if (IsDriveNTFS(otherDrive))
-                return otherDrive;
+            if (!IsDriveNTFS(otherDrive))
+                continue;
+
+            // Filter out drives that don't support volume mount point operations
+            // (e.g., SUBST drives, Azure resource disks without volume GUIDs).
+            // These report as Fixed/NTFS/Ready but GetVolumeNameForVolumeMountPoint fails.
+            if (!HasVolumeGuid(otherDrive))
+                continue;
+
+            return otherDrive;
         }
 
         return null;
     }
 
+    private static bool HasVolumeGuid(string drive)
+    {
+        char[] volumeName = new char[260];
+        return DllImports.GetVolumeNameForVolumeMountPoint(drive, volumeName, volumeName.Length);
+    }
+
     public static string GetNonNtfsDriveOtherThanCurrent()
     {
         return GetNonNtfsDriveOtherThan(GetCurrentDrive());

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj

@@ -42,6 +42,7 @@
     <Compile Include="FileStream\ctor_options.cs" />
     <Compile Include="FileStream\Handle.cs" />
     <Compile Include="Directory\GetLogicalDrives.cs" />
+    <Compile Include="DumpDriveInformation.cs" />
     <Compile Include="FileStream\LockUnlock.cs" />
     <Compile Include="FileSystemTest.cs" />
     <Compile Include="File\AppendAllBytesAsync.cs" />