Apply council review: validate outputs in concurrent tests, compare items in parity tests

- Concurrent tests now validate per-thread output values (not just exception-free)
- Parity tests compare output item counts (ProcessedContentItems, CopyLocalItems, FileWrites, ResolvedAssets)
- ProduceContentAssets_EmptyDependencies_Succeeds now sets TaskEnvironment

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

Author: SimaTian

src/Tasks/Microsoft.NET.Build.Tasks.UnitTests/GivenAProduceContentAssetsMultiThreading.cs

@@ -114,6 +114,12 @@ public void ItProducesSameResultsRegardlessOfCwd()
                     "task should return the same result regardless of CWD");
                 engine1.Errors.Count.Should().Be(engine2.Errors.Count,
                     "error count should be the same in both environments");
+                task1.ProcessedContentItems.Length.Should().Be(task2.ProcessedContentItems.Length,
+                    "ProcessedContentItems count should be identical regardless of CWD");
+                task1.CopyLocalItems.Length.Should().Be(task2.CopyLocalItems.Length,
+                    "CopyLocalItems count should be identical regardless of CWD");
+                task1.FileWrites.Length.Should().Be(task2.FileWrites.Length,
+                    "FileWrites count should be identical regardless of CWD");
             }
             finally
             {

src/Tasks/Microsoft.NET.Build.Tasks.UnitTests/GivenAResolveCopyLocalAssetsMultiThreading.cs

@@ -109,6 +109,8 @@ public void ItProducesSameResultsInMultiProcessAndMultiThreadedEnvironments()
                     "task should return the same result regardless of CWD");
                 engine1.Errors.Count.Should().Be(engine2.Errors.Count,
                     "error count should be identical in both environments");
+                task1.ResolvedAssets.Length.Should().Be(task2.ResolvedAssets.Length,
+                    "ResolvedAssets count should be identical regardless of CWD");
             }
             finally
             {

src/Tasks/Microsoft.NET.Build.Tasks.UnitTests/GivenAttributeOnlyTasksGroup6.cs

@@ -413,7 +413,8 @@ public void ProduceContentAssets_EmptyDependencies_Succeeds()
             {
                 BuildEngine = new MockBuildEngine(),
                 ContentFileDependencies = Array.Empty<ITaskItem>(),
-                ProjectLanguage = "C#"
+                ProjectLanguage = "C#",
+                TaskEnvironment = TaskEnvironmentHelper.CreateForTest()
             };
 
             var result = task.Execute();
@@ -497,6 +498,11 @@ public async System.Threading.Tasks.Task GetDefaultPlatformTargetForNetFramework
 
                     startGate.Wait();
                     task.Execute();
+
+                    if (task.DefaultPlatformTarget != "AnyCPU")
+                    {
+                        errors.Add($"Thread {idx}: Expected 'AnyCPU' but got '{task.DefaultPlatformTarget}'");
+                    }
                 }
                 catch (Exception ex)
                 {
@@ -536,6 +542,11 @@ public async System.Threading.Tasks.Task GetEmbeddedApphostPaths_ConcurrentExecu
 
                     startGate.Wait();
                     task.Execute();
+
+                    if (task.EmbeddedApphostPaths == null || task.EmbeddedApphostPaths.Length != 1)
+                    {
+                        errors.Add($"Thread {idx}: Expected 1 path but got {task.EmbeddedApphostPaths?.Length}");
+                    }
                 }
                 catch (Exception ex)
                 {
@@ -573,6 +584,11 @@ public async System.Threading.Tasks.Task GetNuGetShortFolderName_ConcurrentExecu
 
                     startGate.Wait();
                     task.Execute();
+
+                    if (task.NuGetShortFolderName != "net8.0")
+                    {
+                        errors.Add($"Thread {idx}: Expected 'net8.0' but got '{task.NuGetShortFolderName}'");
+                    }
                 }
                 catch (Exception ex)
                 {