RoslynCodeTaskFactory: Log MSB3753 when task class does not implement ITask (#13517)

## Summary

`RoslynCodeTaskFactory.CreateTask()` silently returned `null` when the
inline task class did not implement `ITask`, causing the engine to emit
the generic MSB4060 error. `CodeTaskFactory` already checks for this and
emits the more specific MSB3753 (`CodeTaskFactory.NeedsITaskInterface`).

## Changes

- **`RoslynCodeTaskFactory.cs`**: Added null check after
`Activator.CreateInstance(TaskType) as ITask` — logs MSB3753 via the
existing `_log` instance (set during `Initialize`), matching
`CodeTaskFactory` behavior.
- **`RoslynCodeTaskFactory_Tests.cs`**: Added
`ClassDoesNotInheritFromITask` test covering both in-proc and
out-of-proc (`MSBUILDFORCEINLINETASKFACTORIESOUTOFPROC`) paths.

## Context

I noticed this when trying to migrate usage of CodeTaskFactory to
RoslynCodeTaskFactory and checking for coverage parity between these two
factories as part of that.

The `CodeTaskFactory` equivalent test (`CodeTaskFactoryTests.cs`)
already validates this behavior. This was an oversight in
`RoslynCodeTaskFactory` — the two factories should produce the same
diagnostic for this user error.

Author: jankratochvilcz

src/Tasks.UnitTests/RoslynCodeTaskFactory_Tests.cs

@@ -18,6 +18,7 @@
 using VerifyTests;
 using VerifyXunit;
 using Xunit;
+using Xunit.Abstractions;
 
 using static VerifyXunit.Verifier;
 
@@ -32,8 +33,11 @@ public class RoslynCodeTaskFactory_Tests
 
         private readonly VerifySettings _verifySettings;
 
-        public RoslynCodeTaskFactory_Tests()
+        private readonly ITestOutputHelper _testOutput;
+
+        public RoslynCodeTaskFactory_Tests(ITestOutputHelper testOutput)
         {
+            _testOutput = testOutput;
             UseProjectRelativeDirectory("TaskFactorySource");
 
             _verifySettings = new();
@@ -784,6 +788,49 @@ public override bool Execute()
             }
         }
 
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void ClassDoesNotInheritFromITask(bool forceOutOfProc)
+        {
+            const string taskName = "ClassDoesNotInheritFromITask";
+            string unformattedMessage = ResourceUtilities.GetResourceString("CodeTaskFactory.NeedsITaskInterface");
+
+            string projectContent = $$"""
+                <Project>
+                  <UsingTask TaskName="{{taskName}}" TaskFactory="RoslynCodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll">
+                    <Task>
+                      <Code Type="Class">
+                namespace InlineTask
+                {
+                    public class {{taskName}}
+                    {
+                        public bool Execute()
+                        {
+                            return true;
+                        }
+                    }
+                }
+                      </Code>
+                    </Task>
+                  </UsingTask>
+                  <Target Name="Build">
+                    <{{taskName}} />
+                  </Target>
+                </Project>
+                """;
+
+            using TestEnvironment env = TestEnvironment.Create(_testOutput);
+            if (forceOutOfProc)
+            {
+                env.SetEnvironmentVariable("MSBUILDFORCEINLINETASKFACTORIESOUTOFPROC", "1");
+            }
+
+            TransientTestProjectWithFiles proj = env.CreateTestProjectWithFiles(projectContent);
+            MockLogger logger = proj.BuildProjectExpectFailure();
+            logger.AssertLogContains(unformattedMessage);
+        }
+
         [Fact]
         public void EmbedsGeneratedFromSourceFileInBinlog()
         {

src/Tasks/RoslynCodeTaskFactory/RoslynCodeTaskFactory.cs

@@ -141,7 +141,18 @@ public ITask CreateTask(IBuildEngine taskFactoryLoggingHost)
         {
             // The type of the task has already been determined and the assembly is already loaded after compilation so
             // just create an instance of the type and return it.
-            return Activator.CreateInstance(TaskType) as ITask;
+            ITask taskInstance = Activator.CreateInstance(TaskType) as ITask;
+            if (taskInstance is null)
+            {
+                TaskLoggingHelper taskInvocationLog = new TaskLoggingHelper(taskFactoryLoggingHost, _taskName)
+                {
+                    TaskResources = AssemblyResources.PrimaryResources,
+                    HelpKeywordPrefix = "MSBuild."
+                };
+                taskInvocationLog.LogErrorWithCodeFromResources("CodeTaskFactory.NeedsITaskInterface", _taskName);
+            }
+
+            return taskInstance;
         }
 
         /// <inheritdoc cref="ITaskFactory.GetTaskParameters"/>