fix(cpp-qt): Fix enum query parameter serialization for both inline and referenced enums

For enum query parameters, the template was incorrectly using asJsonObject()
which returns {"value": "enumValue"} and then iterating over keys, using
"value" as the parameter name instead of the actual parameter name.

This fix adds special handling for both inline enums (isEnum) and referenced
enums (isEnumRef) to use asJson() directly, which returns the correct enum
string value for URL serialization.

Fixes enum query parameters like ?scope=property instead of ?value=property.

Completes the fix started in PR #21211 which added the asJsonObject() method
to make enum code compile, but the template logic was still incorrect for URL
query parameter serialization.

Note: The petstore samples don't contain enum query parameter tests to
demonstrate this fix (they use string arrays). Future contributors may want
to add enum query parameter examples to better showcase this functionality.

Author: ReallyVirtual

modules/openapi-generator/src/main/resources/cpp-qt-client/api-body.mustache

@@ -344,6 +344,21 @@ void {{classname}}::{{nickname}}({{#allParams}}{{#required}}const {{{dataType}}}
         else
             fullPath.append("?");
         {{^isPrimitiveType}}
+        {{#isEnum}}
+        // For enum parameters, use direct string serialization instead of object iteration
+        QString enumValue = {{paramName}}{{^required}}.value(){{/required}}.asJson();
+        if (!enumValue.isEmpty()) {
+            fullPath.append(QUrl::toPercentEncoding("{{baseName}}")).append("=").append(QUrl::toPercentEncoding(enumValue));
+        }
+        {{/isEnum}}
+        {{#isEnumRef}}
+        // For enum reference parameters, use direct string serialization instead of object iteration
+        QString enumValue = {{paramName}}{{^required}}.value(){{/required}}.asJson();
+        if (!enumValue.isEmpty()) {
+            fullPath.append(QUrl::toPercentEncoding("{{baseName}}")).append("=").append(QUrl::toPercentEncoding(enumValue));
+        }
+        {{/isEnumRef}}
+        {{^isEnum}}{{^isEnumRef}}
         QString paramString = (queryStyle == "form" && {{isExplode}}) ? "" : (queryStyle == "form" && !({{isExplode}})) ? "{{baseName}}"+querySuffix : "";
         QJsonObject parameter = {{paramName}}{{^required}}.value(){{/required}}.asJsonObject();
         qint32 count = 0;
@@ -390,6 +405,7 @@ void {{classname}}::{{nickname}}({{#allParams}}{{#required}}const {{{dataType}}}
             count++;
         }
         fullPath.append(paramString);
+        {{/isEnumRef}}{{/isEnum}}
         {{/isPrimitiveType}}{{#isPrimitiveType}}
         fullPath.append(QUrl::toPercentEncoding("{{baseName}}")).append(querySuffix).append(QUrl::toPercentEncoding(::{{cppNamespace}}::toStringValue({{paramName}}{{^required}}.stringValue(){{/required}})));
 {{/isPrimitiveType}}