Address PR review comments: improve tfm resolution and null handling

- Use TargetFrameworkAttribute.FrameworkDisplayName before falling back to
  RuntimeInformation.FrameworkDescription for <tfm> placeholder
- Omit <asm> and <tfm> keys from replacements when null instead of using
  empty string (prevents path traversal with directory templates)
- Strengthen subdirectory assertion to verify directory name equals 'HangDump'

Author: Evangelink

src/Platform/Microsoft.Testing.Extensions.HangDump/HangDumpProcessLifetimeHandler.cs

@@ -314,12 +314,22 @@ private async Task TakeDumpAsync(IProcess process, CancellationToken cancellatio
             ["pname"] = process.Name,
             ["pid"] = processId,
             ["os"] = ArtifactNamingHelper.GetOperatingSystemName(),
-            ["asm"] = Assembly.GetEntryAssembly()?.GetName().Name ?? string.Empty,
-            ["tfm"] = TargetFrameworkParser.GetShortTargetFramework(RuntimeInformation.FrameworkDescription)
-                ?? string.Empty,
             ["time"] = _clock.UtcNow.ToString("yyyy-MM-dd_HH-mm-ss.fffffff", CultureInfo.InvariantCulture),
         };
 
+        string? asmName = Assembly.GetEntryAssembly()?.GetName().Name;
+        if (asmName is not null)
+        {
+            replacements["asm"] = asmName;
+        }
+
+        string? tfm = TargetFrameworkParser.GetShortTargetFramework(Assembly.GetEntryAssembly()?.GetCustomAttribute<TargetFrameworkAttribute>()?.FrameworkDisplayName)
+            ?? TargetFrameworkParser.GetShortTargetFramework(RuntimeInformation.FrameworkDescription);
+        if (tfm is not null)
+        {
+            replacements["tfm"] = tfm;
+        }
+
         string pattern = _dumpFileNamePattern ?? $"{process.Name}_%p_hang.dmp";
 
         // First resolve <placeholder> templates, then handle legacy %p pattern for backward compatibility.

test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpTests.cs

@@ -159,10 +159,12 @@ public async Task HangDump_TemplateFileNameWithSubdirectory_CreateDump(string tf
             dumpFiles,
             $"Expected single dump file in '{resultDirectory}'\n{testHostResult}'");
 
-        // The dump file should be in a subdirectory (not directly under resultDirectory)
+        // The dump file should be in a subdirectory named after the assembly
         string? parentDir = Path.GetDirectoryName(dumpFile);
         Assert.IsNotNull(parentDir);
         Assert.AreNotEqual(resultDirectory, parentDir, "Dump file should be in a subdirectory created from the <asm> placeholder");
+        Assert.AreEqual("HangDump", Path.GetFileName(parentDir),
+            $"Subdirectory should be named after the assembly ('HangDump'). Actual: {Path.GetFileName(parentDir)}");
     }
 
     [DataRow("Mini")]