chore: import upstream snapshot with attribution

dotnet/tests/Microsoft.Agents.AI.LocalCodeAct.UnitTests/FileMountTests.cs

@@ -0,0 +1,52 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+
+namespace Microsoft.Agents.AI.LocalCodeAct.UnitTests;
+
+public sealed class FileMountTests
+{
+    [Fact]
+    public void Constructor_AssignsProperties()
+    {
+        var tempDir = System.IO.Directory.CreateTempSubdirectory("filemount-test-").FullName;
+        try
+        {
+            var mount = new FileMount(tempDir, "/app/data", FileMountMode.ReadWrite, writeBytesLimit: 1024);
+
+            Assert.Equal(tempDir, mount.HostPath);
+            Assert.Equal("/app/data", mount.MountPath);
+            Assert.Equal(FileMountMode.ReadWrite, mount.Mode);
+            Assert.Equal(1024L, mount.WriteBytesLimit);
+        }
+        finally
+        {
+            System.IO.Directory.Delete(tempDir, recursive: true);
+        }
+    }
+
+    [Fact]
+    public void Constructor_DefaultsAreReadWriteWithNoLimit()
+    {
+        var tempDir = System.IO.Directory.CreateTempSubdirectory("filemount-test-").FullName;
+        try
+        {
+            var mount = new FileMount(tempDir, "/app/data");
+            Assert.Equal(FileMountMode.ReadWrite, mount.Mode);
+            Assert.Null(mount.WriteBytesLimit);
+        }
+        finally
+        {
+            System.IO.Directory.Delete(tempDir, recursive: true);
+        }
+    }
+
+    [Fact]
+    public void Constructor_RequiresPaths()
+    {
+        Assert.Throws<ArgumentException>(() => new FileMount("", "/app/data"));
+        Assert.Throws<ArgumentException>(() => new FileMount("/host/data", ""));
+        _ = Assert.Throws<ArgumentNullException>(() => new FileMount(null!, "/app/data"));
+        _ = Assert.Throws<ArgumentNullException>(() => new FileMount("/host/data", null!));
+    }
+}