fix(multivalue): Wave-2 review fixes (MV-2/MV-4/MV-12)

- MV-2: guard unsupported element type before parse (no misleading re-wrap); add Float round-trip test
- MV-4: carry ElementDataType through the two validation-flatten ResolvedAttribute sites (ManagementActor.HandleValidateTemplate, BundleImporter.BuildFlattenedConfigForValidation) so MV-5 validation sees element type via every entry point
- MV-12: include ElementDataType in TemplateAttribute add/update audit payloads + fix stale docstring

src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs

@@ -524,6 +524,7 @@ public class ManagementActor : ReceiveActor
                 CanonicalName = a.Name,
                 Value = a.Value,
                 DataType = a.DataType.ToString(),
+                ElementDataType = a.ElementDataType?.ToString(),
                 IsLocked = a.IsLocked,
                 DataSourceReference = a.DataSourceReference
             }).ToList(),