review(Core.Abstractions): document ReadEventsAsync continuation contract (OpcUaServer-002 root)

Re-review at 7286d320. Core.Abstractions-009: ReadEventsAsync maxEvents<=0 sentinel now
documents the implementer's continuation-point obligation when a backend cap truncates
(the root of OpcUaServer-002). -010: PollGroupEngineTests pass CancellationToken. Plus
EquipmentTagRefResolver.TryResolve [MaybeNullWhen(false)] NRT cleanup + test.

tests/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions.Tests/PollGroupEngineTests.cs

@@ -65,7 +65,7 @@ public sealed class PollGroupEngineTests
             (h, r, s) => events.Enqueue((h, r, s)));
 
         var handle = engine.Subscribe(["X"], TimeSpan.FromMilliseconds(100));
-        await Task.Delay(500);
+        await Task.Delay(500, TestContext.Current.CancellationToken);
         engine.Unsubscribe(handle);
 
         events.Count.ShouldBe(1);
@@ -108,7 +108,7 @@ public sealed class PollGroupEngineTests
         var afterUnsub = events.Count;
 
         src.Values["X"] = 999;
-        await Task.Delay(400);
+        await Task.Delay(400, TestContext.Current.CancellationToken);
         events.Count.ShouldBe(afterUnsub);
     }
 
@@ -125,7 +125,7 @@ public sealed class PollGroupEngineTests
             minInterval: TimeSpan.FromMilliseconds(200));
 
         var handle = engine.Subscribe(["X"], TimeSpan.FromMilliseconds(5));
-        await Task.Delay(300);
+        await Task.Delay(300, TestContext.Current.CancellationToken);
         engine.Unsubscribe(handle);
 
         // 300 ms window, 200 ms floor, stable value → initial push + at most 1 extra poll.
@@ -243,7 +243,7 @@ public sealed class PollGroupEngineTests
         engine.ActiveSubscriptionCount.ShouldBe(0);
 
         var afterDispose = events.Count;
-        await Task.Delay(300);
+        await Task.Delay(300, TestContext.Current.CancellationToken);
         // After dispose no more events — everything is cancelled.
         events.Count.ShouldBe(afterDispose);
     }
@@ -277,7 +277,7 @@ public sealed class PollGroupEngineTests
 
         var handle = engine.Subscribe(["A"], TimeSpan.FromMilliseconds(50));
         // Allow several poll cycles so a broken implementation would accumulate extra events.
-        await Task.Delay(400);
+        await Task.Delay(400, TestContext.Current.CancellationToken);
         engine.Unsubscribe(handle);
 
         // Only the initial-data push should have fired; subsequent polls with identical