review(AdminUI): fix null-TagConfig crash, CTS leak, unencoded historian tag

Review at HEAD 7286d320. AdminUI-002: IsValidJson null/blank -> friendly error (was
ArgumentNullException). AdminUI-003: DriverStatusPanel Reconnect/Restart dispose CTS (build-
verified, live /run deferred). AdminUI-005: HistorianWonderware picker URL-encodes tag name.
AdminUI-008: Format round-trip test. 001 (script-page authz) + 004 (hub [Authorize]) left
Open as cross-cutting w/ Host/Security.

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/UnsTreeService.cs

@@ -1280,9 +1280,17 @@ public sealed class UnsTreeService(IDbContextFactory<OtOpcUaConfigDbContext> dbF
         return null;
     }
 
-    /// <summary>Returns <c>true</c> if <paramref name="json"/> parses as a well-formed JSON document.</summary>
-    private static bool IsValidJson(string json)
+    /// <summary>Returns <c>true</c> if <paramref name="json"/> parses as a well-formed JSON document.
+    /// Null/blank input is treated as invalid (not well-formed JSON) so every caller gets the friendly
+    /// "not valid JSON" result rather than an unhandled <see cref="ArgumentNullException"/> from
+    /// <c>JsonDocument.Parse(null)</c> (AdminUI-002).</summary>
+    private static bool IsValidJson(string? json)
     {
+        if (string.IsNullOrWhiteSpace(json))
+        {
+            return false;
+        }
+
         try
         {
             using var _ = System.Text.Json.JsonDocument.Parse(json);