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

@@ -76,10 +76,18 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
     }
 
     /// <inheritdoc />
+    /// <remarks>
+    ///     A reverse window (<c>start &gt; end</c>) is refused with <see cref="NotSupportedException"/> before anything is sent
+    ///     and is not recorded as a health failure (Gitea #686): the gateway rejects it <c>InvalidArgument</c>, and the server
+    ///     never asks — <see cref="SupportsReverseRaw"/> is <c>false</c>, so it walks a reverse read back with forward reads.
+    /// </remarks>
     public async Task<HistoryReadResult> ReadRawAsync(
         string fullReference, DateTime startUtc, DateTime endUtc, uint maxValuesPerNode,
         CancellationToken cancellationToken)
     {
+        if (startUtc > endUtc)
+            throw new NotSupportedException("The HistorianGateway reads raw history oldest-first only (start > end).");
+
         try
         {
             // The gateway seam caps with an int; OPC UA hands us a uint, so clamp to int range.
@@ -235,6 +243,14 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         }
     }
 
+    /// <inheritdoc />
+    /// <remarks>
+    ///     <c>false</c> (Gitea #686): the gateway's <c>ReadRaw</c> streams ascending only and rejects <c>end &lt; start</c>
+    ///     (<c>InvalidArgument</c>). The server serves reverse and end-only capped raw reads from bounded forward reads
+    ///     instead, and never sends this source a reverse window (<see cref="ReadRawAsync"/> refuses one unsent).
+    /// </remarks>
+    public bool SupportsReverseRaw => false;
+
     /// <inheritdoc />
     /// <remarks>
     ///     <c>false</c>: the gateway's <c>ReadEvents</c> streams ascending only (Gitea #675). The server therefore refuses an

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbHistorianDataSource.cs

@@ -355,6 +355,10 @@ public sealed class QuestDbHistorianDataSource : IHistorianDataSource
 
     // ------------------------------------------------------------------------------------------ Events
 
+    /// <inheritdoc />
+    /// <remarks>Reverse raw reads are served natively (<c>(end, start]</c>, <c>ORDER BY … DESC</c>; #669/#686).</remarks>
+    public bool SupportsReverseRaw => true;
+
     /// <inheritdoc />
     /// <remarks>Reverse events reads are served natively (<c>ORDER BY … DESC</c>, Gitea #675).</remarks>
     public bool SupportsReverseEvents => true;

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer/SqlServerHistorianDataSource.cs

@@ -312,6 +312,10 @@ public sealed class SqlServerHistorianDataSource : IHistorianDataSource
 
     // ------------------------------------------------------------------------------------------ Events
 
+    /// <inheritdoc />
+    /// <remarks>Reverse raw reads are served natively (<c>(end, start]</c>, <c>ORDER BY … DESC</c>; #669/#686).</remarks>
+    public bool SupportsReverseRaw => true;
+
     /// <inheritdoc />
     /// <remarks>Reverse events reads are served natively (<c>ORDER BY … DESC</c>, Gitea #675).</remarks>
     public bool SupportsReverseEvents => true;