fix(WRK-01): STA pump step refreshes activity to prevent false StaHung

A long legitimate ReadBulk pumped Windows messages without refreshing
LastStaActivityUtc, so the watchdog false-positived StaHung past
HeartbeatStuckCeiling and then silently dropped every reply. PumpPendingMessages()
now calls MarkActivity() after pumping; a genuinely stuck STA (no pumping)
still accrues staleness and faults correctly. No MXAccess parity change.

archreview: WRK-01 (P0). Verified on the Windows host (x86): worker builds
clean, StaRuntimeTests + WorkerPipeSessionTests 33/33 pass.

src/ZB.MOM.WW.MxGateway.Worker.Tests/Sta/StaRuntimeTests.cs

@@ -85,6 +85,34 @@ public sealed class StaRuntimeTests
         Assert.True(updated);
     }
 
+    /// <summary>
+    ///     Verifies that <see cref="StaRuntime.PumpPendingMessages"/> refreshes
+    ///     <see cref="StaRuntime.LastActivityUtc"/>. A long synchronous STA
+    ///     command (for example <c>ReadBulk</c> waiting <c>timeout_ms</c> for
+    ///     the first <c>OnDataChange</c>) invokes the pump step on every wait
+    ///     iteration while it legitimately holds the STA thread; refreshing
+    ///     activity here keeps the watchdog from mistaking a busy STA for a hung
+    ///     one (WRK-01). The runtime is deliberately left unstarted so the only
+    ///     source of activity is the pump call under test, not the idle loop.
+    /// </summary>
+    [Fact]
+    public void PumpPendingMessages_RefreshesLastActivity()
+    {
+        RecordingComApartmentInitializer initializer = new();
+        using StaRuntime runtime = CreateRuntime(initializer);
+        DateTimeOffset before = runtime.LastActivityUtc;
+
+        bool refreshed = SpinWait.SpinUntil(
+            () =>
+            {
+                runtime.PumpPendingMessages();
+                return runtime.LastActivityUtc > before;
+            },
+            TimeSpan.FromSeconds(2));
+
+        Assert.True(refreshed, "PumpPendingMessages must advance LastActivityUtc.");
+    }
+
     /// <summary>Verifies that InvokeAsync faults the returned task when a command raises an exception without stopping the runtime.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]