fix(historian): #685 Gateway native TimeAverage/Total — drop AVEVA's pre-window row, keep Uncertain values

AVEVA answers one extra row for the interval before the window; it was mapped as
a result stamped before StartTime (N+1 values). Rows whose interval lies outside
the requested span (either direction) are now dropped after the drained stream.
An Uncertain (opc 64-191) interval kept no value (opc >= 192 was the only data
test); it now carries its value as UncertainDataSubNormal | Calculated.
Docs: AVEVA's TWA excludes NULL-gap time while the SQL backends bridge it;
end-to-end per-interval values also need historiangw#27.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/GatewayHistorianDataSource.cs

@@ -136,7 +136,8 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
     ///     </para>
     ///     <para>
     ///         <b>TimeAverage / Total</b> stay native AVEVA retrieval modes (<see cref="AggregateModeMapper"/>), stamped with
-    ///         the interval start and the Calculated bit. Everything else is <see cref="HistoryAggregateNotSupportedException"/>
+    ///         the interval start and the Calculated bit; AVEVA's extra row for the interval before the window is dropped and
+    ///         an Uncertain interval keeps its value (#685, <see cref="SampleMapper.ToAggregateSnapshots"/>). Everything else is <see cref="HistoryAggregateNotSupportedException"/>
     ///         (see <see cref="ServedAggregates"/>).
     ///     </para>
     /// </remarks>
@@ -165,7 +166,8 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
                 buckets.Add(bucket);
             }
 
-            var snapshots = SampleMapper.ToAggregateSnapshots(buckets);
+            // Filtered while mapping, after the stream was drained (#685: AVEVA's extra pre-window row).
+            var snapshots = SampleMapper.ToAggregateSnapshots(buckets, startUtc, endUtc);
             RecordOutcome(success: true, error: null);
             return new HistoryReadResult(snapshots, ContinuationPoint: null);
         }