fix: make STA Word queue lifetime safe

DocumentAutomation/tests/DocxWorkbench.Tests/StaWordQueueTests.cs

@@ -0,0 +1,22 @@
+using DocxWorkbench.WordInterop;
+
+namespace DocxWorkbench.Tests;
+
+public sealed class StaWordQueueTests
+{
+    [Fact]
+    public async Task Dispose_DrainsActiveJobAndIsIdempotent()
+    {
+        using var queue = new StaWordQueue(); using var started = new ManualResetEventSlim(false); using var release = new ManualResetEventSlim(false);
+        var job = queue.Enqueue(() => { started.Set(); Assert.Equal(ApartmentState.STA, Thread.CurrentThread.GetApartmentState()); release.Wait(); return 42; }, CancellationToken.None); Assert.True(started.Wait(TimeSpan.FromSeconds(2)));
+        var dispose = Task.Run(queue.Dispose); var completed = await Task.WhenAny(dispose, Task.Delay(100)); Assert.NotSame(dispose, completed); release.Set(); await dispose; Assert.Equal(42, await job); queue.Dispose(); Assert.Throws<ObjectDisposedException>(() => EnqueueAfterDispose(queue));
+    }
+
+    [Fact]
+    public async Task Enqueue_CancellationCompletesCanceledTask()
+    {
+        using var queue = new StaWordQueue(); using var cancellation = new CancellationTokenSource(); cancellation.Cancel(); await Assert.ThrowsAnyAsync<OperationCanceledException>(() => queue.Enqueue(() => 1, cancellation.Token));
+    }
+
+    private static void EnqueueAfterDispose(StaWordQueue queue) => queue.Enqueue(() => 1, CancellationToken.None);
+}