fix: don't propagate isString from enum models to properties

Setting property.isString on enum ref properties caused duplicate
WriteString statements in anyOf models (e.g., IconsSizeParameter).
String enum properties are already handled through the isEnum template
path, not the isString path, so propagating isString is both unnecessary
and harmful.

Co-authored-by: alexaka1 <22166651+alexaka1@users.noreply.github.com>

Author: Copilot

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java

@@ -848,7 +848,9 @@ protected void patchProperty(Map<String, CodegenModel> enumRefs, CodegenModel mo
             property.isFloat = refModel.isFloat;
             property.isDouble = refModel.isDouble;
             property.isDecimal = refModel.isDecimal;
-            property.isString = refModel.isString;
+            // Note: do NOT propagate isString here. String enum properties
+            // are handled through the isEnum template path, not the isString path.
+            // Setting isString would cause duplicate write statements in anyOf models.
         }
 
         this.patchPropertyIsInherited(model, property);