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.

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IHistoryProvider.cs

@@ -92,6 +92,14 @@ public interface IHistoryProvider
     ///     adapters historically treat <c>maxEvents &lt;= 0</c> as a sentinel meaning
     ///     "use the backend's default cap" (see <c>WonderwareHistorianClient</c> /
     ///     <c>HistorianDataSource</c>). The asymmetry is intentional — Core.Abstractions-006.
+    ///
+    ///     <b>Continuation contract when using the sentinel:</b> When <c>maxEvents &lt;= 0</c>
+    ///     the backend applies its own cap. If the backend's cap truncates the result
+    ///     (i.e. more matching events exist beyond the returned set), implementations MUST set
+    ///     <see cref="HistoricalEventsResult.ContinuationPoint"/> to a non-null token so
+    ///     callers can detect truncation and page. A null <c>ContinuationPoint</c> means all
+    ///     matching events were returned — callers rely on this to decide whether to page.
+    ///     (Core.Abstractions-009.)
     /// </param>
     /// <param name="cancellationToken">Request cancellation.</param>
     /// <remarks>