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