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/Sta/StaRuntime.cs

@@ -80,14 +80,24 @@ public sealed class StaRuntime : IDisposable
     public bool IsRunning => startedEvent.IsSet && !stoppedEvent.IsSet;
 
     /// <summary>
-    ///     Pumps any pending Windows messages on the calling thread. Intended
-    ///     for commands that synchronously hold the STA (e.g. ReadBulk) and
-    ///     must allow inbound MXAccess COM events to dispatch while they
-    ///     wait. Callers must already be on the STA; the method is otherwise
-    ///     safe (PeekMessage simply finds no messages).
+    ///     Pumps any pending Windows messages on the calling thread and refreshes
+    ///     the STA activity timestamp. Intended for commands that synchronously
+    ///     hold the STA (e.g. ReadBulk) and must allow inbound MXAccess COM events
+    ///     to dispatch while they wait. Because a long-running command invokes this
+    ///     on every wait iteration, refreshing activity here keeps a busy STA from
+    ///     being mistaken for a hung one: a healthy command that keeps pumping stays
+    ///     fresh past <c>HeartbeatStuckCeiling</c>, while a genuinely stuck STA (no
+    ///     pumping) still accrues staleness and faults correctly. Callers must
+    ///     already be on the STA; the method is otherwise safe (PeekMessage simply
+    ///     finds no messages).
     /// </summary>
     /// <returns>The number of messages pumped.</returns>
-    public int PumpPendingMessages() => messagePump.PumpPendingMessages();
+    public int PumpPendingMessages()
+    {
+        int pumpedMessages = messagePump.PumpPendingMessages();
+        MarkActivity();
+        return pumpedMessages;
+    }
 
     /// <summary>
     /// Starts the STA thread.