fix(historian): #675 review follow-ups — SupportsReverseEvents capability, no catch-as-unsupported

- IHistorianDataSource.SupportsReverseEvents (default false; SqlServer/QuestDB true; Gateway/Null declare false).
  The node manager checks it BEFORE calling the source: an end-only capped events read becomes reverse only on
  a supporting backend (Gateway, the default, keeps the pre-#675 forward [MinValue, EndTime) paging), and an
  explicit reverse read on a non-supporting backend is BadHistoryOperationUnsupported without a call.
- Removed the broad catch (NotSupportedException) in the events path: a NotSupportedException from inside a
  backend is now logged as a backend fault (Error), not an 'unsupported' Warning.
- Null declares the reverse overload itself; guard test: every production IHistorianDataSource declares both
  the reverse overload and the capability (positive-control scan of all four backends).
- Conformance: capability reported + oversized reverse tie cluster (forward over-fetch reversed == reverse
  read order; capped reverse read stopping inside the cluster flags truncation) on SqlServer + QuestDB.
- docs/Historian.md: end-only capped events read is backward on SqlServer/QuestDB, forward on Gateway.

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/EventHistoryPagingOverTheWireTests.cs

@@ -134,6 +134,8 @@ public sealed class EventHistoryPagingOverTheWireTests
             return new HistoricalEventsResult(window.Take(cap).ToList(), window.Count > cap ? new byte[] { 1 } : null);
         }
 
+        public bool SupportsReverseEvents => true;
+
         /// <summary>The #675 overload: reverse is <c>(lo, hi]</c> by (time, id) descending.</summary>
         public async Task<HistoricalEventsResult> ReadEventsAsync(
             string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, bool reverse, CancellationToken ct)