test(sessions): switch SessionWorkerClientFactoryFakeWorkerTests to IAsyncLifetime so its teardown actually runs

xUnit v2 (2.9.3) never invokes IAsyncDisposable.DisposeAsync on a test
class, so the unobserved-fault safety net this class documents — awaiting
every scripted worker task after each test — has been dead code since it
was written. Found via dumpasync on the wedged-testhost investigation: the
NeverReadyWorkerProcessLauncher's Task.Delay(Infinite, _stop.Token) was
still pending (DelayPromiseWithCancellation, detached) minutes after its
test finished, which is only possible if DisposeAsync never ran.

Proven both ways with a throw-probe in DisposeAsync: under IAsyncDisposable
all 3 tests pass (never called); under IAsyncLifetime all 3 fail from the
probe (called after every test). Sweep confirmed this is the only test
class on IAsyncDisposable — all other implementers are helpers disposed
via await using.

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/SessionWorkerClientFactoryFakeWorkerTests.cs

@@ -11,7 +11,7 @@ using ZB.MOM.WW.MxGateway.Tests.TestSupport;
 
 namespace ZB.MOM.WW.MxGateway.Tests.Gateway.Sessions;
 
-public sealed class SessionWorkerClientFactoryFakeWorkerTests : IAsyncDisposable
+public sealed class SessionWorkerClientFactoryFakeWorkerTests : IAsyncLifetime
 {
     private static readonly TimeSpan TestTimeout = TimeSpan.FromSeconds(5);
 
@@ -24,12 +24,19 @@ public sealed class SessionWorkerClientFactoryFakeWorkerTests : IAsyncDisposable
 
     private readonly List<IWorkerTaskLauncher> _launchers = [];
 
+    /// <inheritdoc />
+    public Task InitializeAsync() => Task.CompletedTask;
+
     /// <summary>
     /// Awaits every scripted worker task so an unhandled exception fails the owning test
     /// instead of surfacing later as an unobserved <see cref="TaskScheduler.UnobservedTaskException"/>.
+    /// This must stay on <see cref="IAsyncLifetime"/>: xUnit v2 never invokes
+    /// <see cref="IAsyncDisposable.DisposeAsync"/> on a test class, so under the previous
+    /// declaration this teardown silently never ran (the NeverReady launcher's parked infinite
+    /// delay was still pending in a post-run process dump).
     /// </summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
-    public async ValueTask DisposeAsync()
+    public async Task DisposeAsync()
     {
         foreach (IWorkerTaskLauncher launcher in _launchers)
         {