fix(historian): trim only HistoryServerCapabilities/AggregateFunctions; null set = all (#682 review)

ServerCapabilities/AggregateFunctions (i=2997) is Part 5's list of every
aggregate the server supports, and live AggregateFilter subscriptions still
compute all 37 SDK aggregates, so it keeps the SDK's full set; only i=11201
follows the backend. A null SupportedAggregates is treated as the interface
default (all) by both the advertisement and HistoryRead-Processed (it used to
NRE inside the batch). Docs: i=2997/i=11201 split and the boot window.

Co-Authored-By: Claude <noreply@anthropic.com>

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/Historian/IHistorianDataSource.cs

@@ -176,9 +176,10 @@ public interface IHistorianDataSource : IDisposable
 
     /// <summary>
     ///     The Part 13 aggregates this backend serves for a processed HistoryRead (Gitea #682) — the ONE authority for
-    ///     both what the server advertises under <c>ServerCapabilities/AggregateFunctions</c> (i=2997) and
-    ///     <c>HistoryServerCapabilities/AggregateFunctions</c> (i=11201), and what its HistoryRead-Processed answers
-    ///     <c>BadAggregateNotSupported</c> up front without calling the backend.
+    ///     both what the server advertises under <c>HistoryServerCapabilities/AggregateFunctions</c> (i=11201) and what
+    ///     its HistoryRead-Processed answers <c>BadAggregateNotSupported</c> up front without calling the backend.
+    ///     (<c>ServerCapabilities/AggregateFunctions</c>, i=2997, is not affected: it lists every aggregate the server
+    ///     computes, including for live <c>AggregateFilter</c> subscriptions.)
     /// </summary>
     /// <remarks>
     ///     The default is <see cref="HistoryAggregateTypes.All"/> — "every aggregate the server can name; a member I cannot
@@ -187,7 +188,8 @@ public interface IHistorianDataSource : IDisposable
     ///     <c>null</c> "all" sentinel so every consumer handles one shape. Every PRODUCTION backend declares its set itself
     ///     (a reflection guard pins that): a wrapper that forgot to forward this would otherwise advertise every aggregate.
     ///     The set is read once when the source is wired (the backend is fixed at boot by <c>ServerHistorian:Backend</c>)
-    ///     and again on every processed read, so it must be cheap and stable — return a static frozen set.
+    ///     and again on every processed read, so it must be cheap and stable — return a static frozen set. A source that
+    ///     returns <c>null</c> anyway is treated as the default (all) by both consumers.
     /// </remarks>
     IReadOnlySet<HistoryAggregateType> SupportedAggregates => HistoryAggregateTypes.All;