fix: accept BackupOrRestore only from .NET 9, as the runtime does

.NET 8's options check rejects `FileOptions` 0x02000000; .NET 9 and later
accept it. The mock now does the same on each target.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TymoqrVYgZwdNSper5ZZB1

Author: mpdreamz

src/TestableIO.System.IO.Abstractions.TestingHelpers/MockSafeFileHandles.cs

@@ -30,8 +30,10 @@ internal sealed class MockSafeFileHandles
                                                  FileOptions.DeleteOnClose |
                                                  FileOptions.SequentialScan |
                                                  FileOptions.Encrypted |
-                                                 (FileOptions)0x20000000 | // NoBuffering
-                                                 (FileOptions)0x02000000; // BackupOrRestore
+#if NET9_0_OR_GREATER
+                                                 (FileOptions)0x02000000 | // BackupOrRestore, accepted since .NET 9
+#endif
+                                                 (FileOptions)0x20000000; // NoBuffering
 
     private readonly IMockFileDataAccessor mockFileDataAccessor;
     private readonly Dictionary<IntPtr, Entry> entries = new();

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockFileOpenHandleTests.cs

@@ -143,7 +143,9 @@ await That(() => fileSystem.File.OpenHandle(FilePath, FileMode.Create, FileAcces
             .Throws<ArgumentOutOfRangeException>().WithParamName("preallocationSize");
     }
 
+#if NET9_0_OR_GREATER
     [TestCase((FileOptions)0x02000000)]
+#endif
     [TestCase((FileOptions)0x20000000)]
     [TestCase(FileOptions.Asynchronous | FileOptions.WriteThrough)]
     public async Task OpenHandle_WithAnOptionTheRuntimeAccepts_ShouldOpen(FileOptions options)
@@ -158,6 +160,20 @@ await That(() =>
         }).DoesNotThrow();
     }
 
+#if !NET9_0_OR_GREATER
+    [Test]
+    public async Task OpenHandle_WithBackupOrRestoreBeforeNet9_ShouldThrowArgumentOutOfRangeException()
+    {
+        var fileSystem = new MockFileSystem();
+        fileSystem.AddFile(FilePath, new MockFileData("content"));
+
+        await That(() => fileSystem.File.OpenHandle(FilePath, FileMode.Open, FileAccess.Read, FileShare.Read,
+                (FileOptions)0x02000000))
+            .Throws<ArgumentOutOfRangeException>().WithParamName("options")
+            .Because("the runtime accepts BackupOrRestore only since .NET 9");
+    }
+
+#endif
     [Test]
     public async Task OpenHandle_PreallocationSizeOnExistingFile_ShouldThrowArgumentException()
     {