perf(drivers): fixed-rate poll pacing with overrun counter; non-blocking unsubscribe

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -1341,13 +1341,14 @@ public sealed class ModbusDriver
         Task.FromResult(_poll.Subscribe(fullReferences, publishingInterval));
 
     /// <inheritdoc />
-    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
-        _poll.Unsubscribe(handle);
+        // Async teardown: the engine drains the in-flight poll without blocking the caller's
+        // thread — this runs inside a DriverInstanceActor mailbox turn.
+        await _poll.UnsubscribeAsync(handle).ConfigureAwait(false);
         // Drop any partially-collected cycle buffer so a torn-down subscription leaks nothing.
-        // Unsubscribe awaits the loop task, so no onChange can re-add after this point.
+        // The teardown above awaits the loop task, so no onChange can re-add after this point.
         _cycleBatchItems.TryRemove(handle, out _);
-        return Task.CompletedTask;
     }
 
     // ---- IHostConnectivityProbe ----