Fix review issues: parent constructor call and hasVars/hasRequired flags

Address two issues identified by cubic code review:

1. Always emit parentheses in parent constructor call when
   `useSealedDiscriminatorClasses` is enabled, even when
   `x-parent-ctor-args` is absent. This prevents invalid Kotlin like
   `: Parent` when the parent is a sealed class (should be `: Parent()`).

2. Update `hasVars` and `hasRequired` flags on child models after adding
   discriminator properties in the oneOf pattern. Without this, a child
   with no other properties would generate `class Cat(...)` instead of
   `data class Cat(...)`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rvlasveld

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

@@ -1394,6 +1394,10 @@ public Map<String, ModelsMap> postProcessAllModels(Map<String, ModelsMap> objs)
                                     childModel.getAllVars().add(discriminatorProp);
                                 }
 
+                                // Update model flags after adding discriminator property
+                                childModel.setHasVars(true);
+                                childModel.setHasRequired(true);
+
                                 childModel.getVendorExtensions().put("x-parent-ctor-args",
                                         discriminatorVarName + " = " + discriminatorVarName);
                             }

modules/openapi-generator/src/main/resources/kotlin-spring/dataClass.mustache

@@ -27,7 +27,7 @@
 {{/hasOptional}}{{/hasRequired}}{{#optionalVars}}{{>dataClassOptVar}}{{^-last}},
 {{/-last}}{{/optionalVars}}
 ){{/discriminator}}{{! no newline
-}}{{#parent}} : {{{.}}}{{#isMap}}(){{/isMap}}{{^isMap}}{{#vendorExtensions.x-parent-ctor-args}}({{{vendorExtensions.x-parent-ctor-args}}}){{/vendorExtensions.x-parent-ctor-args}}{{/isMap}}{{! no newline
+}}{{#parent}} : {{{.}}}{{#isMap}}(){{/isMap}}{{^isMap}}{{#useSealedDiscriminatorClasses}}({{#vendorExtensions.x-parent-ctor-args}}{{{vendorExtensions.x-parent-ctor-args}}}{{/vendorExtensions.x-parent-ctor-args}}){{/useSealedDiscriminatorClasses}}{{/isMap}}{{! no newline
       }}{{#vendorExtensions.x-kotlin-implements}}, {{{.}}}{{/vendorExtensions.x-kotlin-implements}}{{! <- serializableModel is also handled via x-kotlin-implements
       }}{{#vendorExtensions.x-implements-sealed-interfaces}}{{#.}}, {{{.}}}{{/.}}{{/vendorExtensions.x-implements-sealed-interfaces}}{{! <- add sealed interface implementations
 }}{{/parent}}{{! no newline