test(galaxy): readback via explicit TCS + skip unused buffered-interval RPC (review)

Code-review refinement of the live-gw read-back helper: complete a
TaskCompletionSource<double?> from the pump instead of a captured local (explicit
cross-task visibility), pass bufferedUpdateIntervalMs:0 (Advise snapshot needs no
SetBufferedUpdateInterval), and document the Advise->OnDataChange filter. Live re-verified 2/2.

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GatewayGalaxyLiveReopenAndWriteTests.cs

@@ -155,7 +155,9 @@ public sealed class GatewayGalaxyLiveReopenAndWriteTests
         await session.ConnectAsync(clientOptions, ct);
         var subscriber = new GatewayGalaxySubscriber(session);
 
-        double? captured = null;
+        // First decoded data-change value, completed by the pump (an explicit TCS rather than a
+        // captured local, so the value's visibility across the Task boundary is unambiguous).
+        var firstValue = new TaskCompletionSource<double?>(TaskCreationOptions.RunContinuationsAsynchronously);
         using var streamCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
         streamCts.CancelAfter(timeout);
 
@@ -165,20 +167,26 @@ public sealed class GatewayGalaxyLiveReopenAndWriteTests
             {
                 await foreach (var ev in subscriber.StreamEventsAsync(streamCts.Token))
                 {
+                    // SubscribeBulk uses Advise (not AdviseBuffered), so the initial galaxy-value
+                    // snapshot arrives as OnDataChange — never OnBufferedDataChange.
                     if (ev.Family != MxEventFamily.OnDataChange) continue;
                     if (MxValueDecoder.Decode(ev.Value) is not { } decoded) continue;
-                    captured = Convert.ToDouble(decoded);
-                    streamCts.Cancel();
+                    firstValue.TrySetResult(Convert.ToDouble(decoded));
                     return;
                 }
             }
-            catch (OperationCanceledException) { /* timeout, or we got the value and cancelled */ }
+            catch (OperationCanceledException) { /* timeout, or stream ended */ }
+            firstValue.TrySetResult(null); // timed out / stream completed without a data event
         }, ct);
 
-        // Let the StreamEvents RPC attach before adding the subscription so the initial snapshot isn't missed.
+        // Let the StreamEvents RPC attach before adding the subscription so the initial snapshot isn't
+        // missed. bufferedUpdateIntervalMs:0 skips the superfluous SetBufferedUpdateInterval RPC — the
+        // Advise-based snapshot is delivered regardless.
         await Task.Delay(250, ct);
-        await subscriber.SubscribeBulkAsync([fullRef], bufferedUpdateIntervalMs: 500, ct);
-        await pump;
-        return captured;
+        await subscriber.SubscribeBulkAsync([fullRef], bufferedUpdateIntervalMs: 0, ct);
+        var result = await firstValue.Task;
+        streamCts.Cancel(); // stop the pump's StreamEvents enumeration
+        await pump;         // observe any pump exception + ensure teardown before the session disposes
+        return result;
     }
 }