fix(historian): #686 serve reverse/end-only raw reads on forward-only backends (Gateway)

#669 turned an end-only capped HistoryRead-Raw into a reverse read (start > end)
for every backend; the gateway rejects start > end (InvalidArgument), so on the
default Gateway backend every end-only capped and every explicit reverse raw read
failed the node and was counted as a backend health failure.

- IHistorianDataSource.SupportsReverseRaw (default false; SqlServer/QuestDB/Null
  true, Gateway false; guard test pins that every production backend declares it).
- OtOpcUaNodeManager sends a reverse window only to a backend that declares it;
  otherwise ReverseRawReader walks (lo, hi] back in bounded forward segment reads
  (1 min, x4 while sparse; each <= MaxTieClusterOverfetch + 1 rows; a full read is
  re-read narrower, never answered truncated; filtered on sample timestamps, which
  also drops AVEVA's fabricated query-start rows; stops at the Unix epoch). A
  one-timestamp tie cluster over the bound => BadTooManyOperations.
- ServeRawPagedAsync maps HistoryReadLimitExceededException to BadTooManyOperations.
- GatewayHistorianDataSource.ReadRawAsync refuses start > end unsent, without
  recording a health failure.

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

@@ -17,6 +17,12 @@ public interface IHistorianDataSource : IDisposable
     /// <summary>
     ///     Read raw historical samples for a single tag over a time range.
     /// </summary>
+    /// <remarks>
+    ///     Forward (<c>startUtc &lt; endUtc</c>) is <c>[start, end)</c> ascending; <c>startUtc == endUtc</c> is a point read.
+    ///     <c>startUtc &gt; endUtc</c> is the Part 11 reverse read <c>(end, start]</c> newest-first — but only a backend that
+    ///     declares <see cref="SupportsReverseRaw"/> is ever sent one (Gitea #686); the server serves a reverse read on any
+    ///     other backend from bounded forward reads.
+    /// </remarks>
     /// <param name="fullReference">The full reference of the tag to read.</param>
     /// <param name="startUtc">The start of the time range in UTC.</param>
     /// <param name="endUtc">The end of the time range in UTC.</param>
@@ -30,6 +36,16 @@ public interface IHistorianDataSource : IDisposable
         uint maxValuesPerNode,
         CancellationToken cancellationToken);
 
+    /// <summary>
+    ///     Whether this backend can serve a reverse (newest-first) raw read itself — <see cref="ReadRawAsync"/> with
+    ///     <c>startUtc &gt; endUtc</c> (Gitea #686). The server checks it before every raw read. When <c>false</c> it never
+    ///     sends <c>start &gt; end</c>: it serves the reverse read — an explicit one, a resumed reverse page, and the
+    ///     end-only capped read Part 11 reads backward from EndTime — from a series of bounded FORWARD reads walking back
+    ///     from the later bound (<c>ReverseRawReader</c> in the OPC UA server). Default <c>false</c>, the safe direction: a
+    ///     source that only reads forward is still answered correctly. A backend declares it itself (a guard test pins that).
+    /// </summary>
+    bool SupportsReverseRaw => false;
+
     /// <summary>
     ///     Read processed (interval-bucketed) samples — average / min / max / count / etc.
     ///     A bucket with no source data returns a sample whose

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

@@ -55,6 +55,11 @@ public sealed class NullHistorianDataSource : IHistorianDataSource
         int maxEvents,
         CancellationToken cancellationToken) => Task.FromResult(EmptyEvents);
 
+    /// <inheritdoc />
+    /// <remarks><c>true</c> (Gitea #686): there is no history, so the empty result is the correct answer in either
+    /// direction — no need for the server to walk back through forward reads to find nothing.</remarks>
+    public bool SupportsReverseRaw => true;
+
     /// <inheritdoc />
     /// <remarks><c>false</c> (Gitea #675): there is no history to read in either direction, and an explicit reverse read
     /// is answered <c>BadHistoryOperationUnsupported</c> like any backend that cannot read newest-first.</remarks>