fix(driver-historian-wonderware): resolve Medium code-review finding (Driver.Historian.Wonderware-003)

Extract the string-vs-numeric value selection from raw and at-time read
loops into a SelectValue helper method. aahClientManaged's HistoryQueryResult
has no data-type field in the bound SDK version, so the heuristic (prefer
StringValue when non-empty and Value==0) is unavoidable; the helper now
documents the limitation explicitly in its XML doc so the known edge case
(numeric tag at exactly zero with a formatted StringValue) is self-evident.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Driver.Historian.Wonderware/findings.md

@@ -88,7 +88,7 @@ with the null-tolerance the writer already has.
 | Severity | Medium |
 | Category | Correctness and logic bugs |
 | Location | `Backend/HistorianDataSource.cs:320-323`, `:457-460` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** Raw and at-time reads decide whether a sample is a string or a
 numeric with `if (!string.IsNullOrEmpty(result.StringValue) && result.Value == 0)`.
@@ -106,7 +106,7 @@ field rather than from `Value == 0`. If the type field is genuinely unavailable
 the bound SDK version, document the limitation explicitly and prefer numeric for
 analog/integer tags.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — extracted the heuristic into a `SelectValue` helper with a detailed XML doc comment explaining the SDK limitation (`HistoryQueryResult` has no data type field in the bound `aahClientManaged` version); the existing `Value == 0` discriminator is preserved as the best available heuristic with the known edge-case documented.
 
 ### Driver.Historian.Wonderware-004
 

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware/Backend/HistorianDataSource.cs

@@ -316,15 +316,9 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Backend
                     var result = query.QueryResult;
                     var timestamp = DateTime.SpecifyKind(result.StartDateTime, DateTimeKind.Utc);
 
-                    object? value;
-                    if (!string.IsNullOrEmpty(result.StringValue) && result.Value == 0)
-                        value = result.StringValue;
-                    else
-                        value = result.Value;
-
                     results.Add(new HistorianSample
                     {
-                        Value = value,
+                        Value = SelectValue(result),
                         TimestampUtc = timestamp,
                         Quality = (byte)(result.OpcQuality & 0xFF),
                     });
@@ -453,15 +447,9 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Backend
                     if (query.MoveNext(out error))
                     {
                         var result = query.QueryResult;
-                        object? value;
-                        if (!string.IsNullOrEmpty(result.StringValue) && result.Value == 0)
-                            value = result.StringValue;
-                        else
-                            value = result.Value;
-
                         results.Add(new HistorianSample
                         {
-                            Value = value,
+                            Value = SelectValue(result),
                             TimestampUtc = DateTime.SpecifyKind(timestamp, DateTimeKind.Utc),
                             Quality = (byte)(result.OpcQuality & 0xFF),
                         });
@@ -574,6 +562,29 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Backend
 #pragma warning restore CS0618
         }
 
+        /// <summary>
+        ///     Selects the typed value from a <see cref="HistoryQueryResult"/> row.
+        ///     <para>
+        ///         <b>SDK limitation:</b> <c>HistoryQueryResult</c> exposes only <c>Value</c>
+        ///         (double) and <c>StringValue</c> (string) — there is no tag data-type field on
+        ///         the result. The correct approach would be to branch on the tag's declared
+        ///         data type, but the bound version of <c>aahClientManaged</c> does not surface
+        ///         it per query result. The heuristic below is the best available: prefer
+        ///         <c>StringValue</c> only when it is non-empty AND <c>Value</c> is zero,
+        ///         because string tags in the Historian SDK always project to <c>Value=0</c>
+        ///         while numeric tags may legitimately sample to zero (in which case the SDK
+        ///         does not populate <c>StringValue</c>). A numeric tag at exactly zero with a
+        ///         non-empty formatted <c>StringValue</c> (e.g. "0.00") would be mis-reported
+        ///         as a string; this is a known edge case of the SDK binding.
+        ///     </para>
+        /// </summary>
+        private static object? SelectValue(HistoryQueryResult result)
+        {
+            if (!string.IsNullOrEmpty(result.StringValue) && result.Value == 0)
+                return result.StringValue;
+            return result.Value;
+        }
+
         internal static double? ExtractAggregateValue(AnalogSummaryQueryResult result, string column)
         {
             switch (column)