[Bug][kotlin-spring] add a Spring type converter for enum values #21564 (#21579)

* [kotlin-spring] add a Spring type converter for enum values #21564

* [kotlin-spring] simplify unit test for inner enum converter

* update samples

* code review feedback; move containsEnums to ModelUtils

* code review feedback; provide comment for generated EnumConverterConfiguration.kt

* update samples

---------

Co-authored-by: Chris Gual <cgual@omnidian.com>

samples/server/petstore/kotlin-springboot-integer-enum/.openapi-generator/FILES

@@ -9,5 +9,6 @@ settings.gradle
 src/main/kotlin/org/openapitools/api/ApiUtil.kt
 src/main/kotlin/org/openapitools/api/DefaultApi.kt
 src/main/kotlin/org/openapitools/api/Exceptions.kt
+src/main/kotlin/org/openapitools/configuration/EnumConverterConfiguration.kt
 src/main/kotlin/org/openapitools/model/ApiError.kt
 src/main/kotlin/org/openapitools/model/ReasonCode.kt

samples/server/petstore/kotlin-springboot-integer-enum/src/main/kotlin/org/openapitools/configuration/EnumConverterConfiguration.kt

@@ -0,0 +1,26 @@
+package org.openapitools.configuration
+
+import org.openapitools.model.ReasonCode
+
+import org.springframework.context.annotation.Bean
+import org.springframework.context.annotation.Configuration
+import org.springframework.core.convert.converter.Converter
+
+/**
+ * This class provides Spring Converter beans for the enum models in the OpenAPI specification.
+ *
+ * By default, Spring only converts primitive types to enums using Enum::valueOf, which can prevent
+ * correct conversion if the OpenAPI specification is using an `enumPropertyNaming` other than
+ * `original` or the specification has an integer enum.
+ */
+@Configuration(value = "org.openapitools.configuration.enumConverterConfiguration")
+class EnumConverterConfiguration {
+
+    @Bean(name = ["org.openapitools.configuration.EnumConverterConfiguration.reasonCodeConverter"])
+    fun reasonCodeConverter(): Converter<kotlin.Int, ReasonCode> {
+        return object: Converter<kotlin.Int, ReasonCode> {
+            override fun convert(source: kotlin.Int): ReasonCode = ReasonCode.forValue(source)
+        }
+    }
+
+}