feat(historian): advertise only the backend's aggregates under AggregateFunctions (#682)

IHistorianDataSource.SupportedAggregates (default: every HistoryAggregateType)
is the one authority for both ServerCapabilities/AggregateFunctions (i=2997) and
HistoryServerCapabilities/AggregateFunctions (i=11201) and for the up-front
BadAggregateNotSupported in HistoryRead-Processed. Gateway declares
AggregateModeMapper.Served (4), SqlServer/QuestDB AggregateFamilies.Served (17),
Null none. Only the folder references are trimmed; the SDK's calculator
factories stay so live AggregateFilter subscriptions keep working.

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

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

@@ -55,6 +55,18 @@ public enum HistoryAggregateType
     Total2,
 }
 
+/// <summary>Shared <see cref="HistoryAggregateType"/> sets for <see cref="IHistorianDataSource.SupportedAggregates"/> (#682).</summary>
+public static class HistoryAggregateTypes
+{
+    /// <summary>Every defined <see cref="HistoryAggregateType"/> — the interface default.</summary>
+    public static readonly IReadOnlySet<HistoryAggregateType> All =
+        System.Collections.Frozen.FrozenSet.ToFrozenSet(Enum.GetValues<HistoryAggregateType>());
+
+    /// <summary>No aggregates — a source with no history to aggregate (the Null historian).</summary>
+    public static readonly IReadOnlySet<HistoryAggregateType> None =
+        System.Collections.Frozen.FrozenSet<HistoryAggregateType>.Empty;
+}
+
 /// <summary>
 ///     Thrown by a history backend when it cannot serve the requested <see cref="HistoryAggregateType"/>.
 ///     The Server HistoryRead path translates it to <c>BadAggregateNotSupported</c> for that node only.

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

@@ -174,6 +174,23 @@ public interface IHistorianDataSource : IDisposable
     /// </remarks>
     long TimestampResolutionTicks => 1;
 
+    /// <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.
+    /// </summary>
+    /// <remarks>
+    ///     The default is <see cref="HistoryAggregateTypes.All"/> — "every aggregate the server can name; a member I cannot
+    ///     serve throws <see cref="HistoryAggregateNotSupportedException"/> at read time", which is exactly the pre-#682
+    ///     behaviour, so a test fake or an out-of-tree source keeps working unchanged. It is a concrete set rather than a
+    ///     <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.
+    /// </remarks>
+    IReadOnlySet<HistoryAggregateType> SupportedAggregates => HistoryAggregateTypes.All;
+
     /// <summary>
     ///     Read modified-value history — OPC UA HistoryReadRaw with <c>IsReadModified = true</c> (Part 11, Gitea #668):
     ///     every modification a HistoryUpdate applied to the tag inside the window, each carrying the PRIOR value.

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

@@ -74,6 +74,15 @@ public sealed class NullHistorianDataSource : IHistorianDataSource
             ? throw new NotSupportedException("The null historian cannot read events newest-first (reverse).")
             : Task.FromResult(EmptyEvents);
 
+    /// <inheritdoc />
+    /// <remarks>
+    ///     EMPTY (Gitea #682): with no historian configured the server advertises no aggregate functions, and a processed
+    ///     HistoryRead answers <c>BadAggregateNotSupported</c> per node rather than <c>GoodNoData</c> — an aggregate is a
+    ///     statistic the server cannot compute without a history, and advertising the SDK's full list on a server with no
+    ///     historian misled capability discovery. Raw / AtTime / Events reads still return the empty <c>GoodNoData</c> result.
+    /// </remarks>
+    public IReadOnlySet<HistoryAggregateType> SupportedAggregates => HistoryAggregateTypes.None;
+
     /// <inheritdoc />
     public HistorianHealthSnapshot GetHealthSnapshot() => new(
         TotalQueries: 0,