tiny bugfix for nullable types in openapi-generator to make the tests pass unchanged

Author: Jolanrensen

dataframe-openapi-generator/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/readOpenapi.kt

@@ -332,7 +332,12 @@ private fun Schema<*>.toMarker(
 
                         fields += allSuperFields
                             .filter {
-                                it.fieldName.unquoted in requiredFields && it.fieldType.isNullable()
+                                it.fieldName.unquoted in requiredFields &&
+                                    it.fieldType.isNullable() &&
+                                    // Don't override intrinsically nullable types (like nullable enums) to be
+                                    // non-null. Their nullability comes from the type itself, not from being
+                                    // optional, so being required in a child schema cannot make them non-null.
+                                    !it.fieldType.refersToNullableMarker(getRefMarker, topInterfaceName)
                             }.map {
                                 generatedFieldOf(
                                     fieldName = it.fieldName,

dataframe-openapi-generator/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/utils.kt

@@ -3,6 +3,7 @@ package org.jetbrains.kotlinx.dataframe.io
 import org.jetbrains.kotlinx.dataframe.codeGen.FieldType
 import org.jetbrains.kotlinx.dataframe.codeGen.GeneratedField
 import org.jetbrains.kotlinx.dataframe.codeGen.ValidFieldName
+import org.jetbrains.kotlinx.dataframe.codeGen.name
 import org.jetbrains.kotlinx.dataframe.impl.toCamelCaseByDelimiters
 import org.jetbrains.kotlinx.dataframe.schema.ColumnSchema
 import kotlin.reflect.typeOf
@@ -23,6 +24,21 @@ internal fun String.snakeToUpperCamelCase(): String =
 
 internal fun String.toNullable() = if (this.last() == '?') this else "$this?"
 
+/**
+ * Returns whether this [FieldType] refers to a [Marker][OpenApiMarker] that is itself nullable
+ * (such as a nullable enum, whose nullability is defined by the type and cannot be removed by
+ * making a property required). Returns `false` for primitives and non-nullable markers.
+ */
+internal fun FieldType.refersToNullableMarker(getRefMarker: GetRefMarker, topInterfaceName: ValidFieldName): Boolean {
+    val refName = name
+        .removeSuffix("?")
+        .withoutTopInterfaceName(topInterfaceName)
+    return when (val res = getRefMarker(refName)) {
+        is MarkerResult.OpenApiMarker -> res.marker.nullable
+        else -> false
+    }
+}
+
 internal interface IsObject {
     val isObject: Boolean
 }