test(drivers): tighten driver-page converter guard + clarify S7 numeric case

Code-review follow-ups: the page-coverage test now asserts every *DriverPage
declares a _jsonOpts serializer (so a new page that serialises config a
different way fails the guard, not just converter removal); clarify that 40 ==
(int)S7CpuType.S71500 in the numeric-throws test.

tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/DriverPageJsonConverterTests.cs

@@ -53,9 +53,17 @@ public sealed class DriverPageJsonConverterTests
             "enum config fields serialise as numbers and the string-typed driver factory throws on parse.");
     }
 
-    /// <summary>Sanity-check that reflection actually discovered the full driver-page fleet (so a
-    /// rename/move can't silently shrink the guarded set to zero).</summary>
+    /// <summary>Enforces that EVERY concrete <c>*DriverPage</c> routes config serialization through a
+    /// <c>_jsonOpts</c> field — otherwise a new page that serialised config a different way would slip
+    /// past the converter guard above. Also a floor check so a rename can't silently shrink the set.</summary>
     [Fact]
-    public void All_known_driver_pages_are_covered()
-        => DriverPageTypes.Count.ShouldBeGreaterThanOrEqualTo(9);
+    public void Every_driver_page_uses_a_guarded_jsonOpts_serializer()
+    {
+        var allDriverPages = typeof(S7DriverPage).Assembly.GetTypes()
+            .Where(t => t.Name.EndsWith("DriverPage", StringComparison.Ordinal) && !t.IsAbstract)
+            .ToList();
+        allDriverPages.Count.ShouldBeGreaterThanOrEqualTo(9, "reflection should discover the full driver-page fleet");
+        DriverPageTypes.Count.ShouldBe(allDriverPages.Count,
+            "every *DriverPage must declare a _jsonOpts config serializer so the string-enum converter guard covers it");
+    }
 }