fix(historian): #691.4 review — uncapped Gateway events reads no longer pre-size to int.MaxValue

GatewayHistorianDataSource.ReadEventsAsync sized its result list to
maxEvents, which is int.MaxValue for NumValuesPerNode = 0: every
uncapped Gateway events read (forward on master, and the new walked
reverse one) threw 'Array dimensions exceeded' and counted as a backend
failure. Capacity is bounded at 1024. The events walk delegate moved to
ReverseEventsReader so it is tested over the real Gateway source
(forward uncapped, reverse uncapped, reverse capped).

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/FakeHistorianGatewayClient.cs

@@ -54,6 +54,9 @@ public sealed class FakeHistorianGatewayClient : IHistorianGatewayClient
     public int LastReadEventsMaxEvents;
     public int ReadEventsCallCount;
 
+    /// <summary>When set, ReadEvents serves <c>EventsSource(start, end)</c> instead of <see cref="Events"/>.</summary>
+    public Func<DateTime, DateTime, IEnumerable<HistorianEvent>>? EventsSource;
+
     /// <summary>Thrown (deferred to first enumeration) by every read method when set.</summary>
     public Exception? ThrowOnRead;
 
@@ -187,7 +190,7 @@ public sealed class FakeHistorianGatewayClient : IHistorianGatewayClient
         LastReadEventsEndUtc = endUtc;
         LastReadEventsMaxEvents = maxEvents;
         ReadEventsCallCount++;
-        return ToAsyncStream(Events, ThrowOnRead, ct);
+        return ToAsyncStream(EventsSource is null ? Events : EventsSource(startUtc, endUtc).ToList(), ThrowOnRead, ct);
     }
 
     public Task<WriteAck> WriteLiveValuesAsync(