[BUG][Kotlin] Multiplatform serializes enum constant name instead of value

[brandon-level]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator? No validation issues detected.
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

When generating Kotlin multiplatform code, an enum defined in /components/schemas using x-enum-varnames generates an enum with toString() overridden to return its value, like in this abbreviated example:

@Serializable
enum class AllowFoo(val value: kotlin.String) {
    @SerialName(value = "D")
    DISALLOW("D"),

    @SerialName(value = "A")
    ALLOW("A");

    override fun toString(): kotlin.String = value
}

But when an enum is declared inline, the generated class does not override toString() and it serializes to the constant's name in code. In the example spec (link below,) the X-Allow-Foo-Ref header declaration generates XAllowFooExampleGet, whose values serialize to "DISALLOW" and "ALLOW" respectively, instead of "D" and "A".

openapi-generator version

Discovered in 7.21.0, verified in master.

OpenAPI declaration file content or url

See https://gist.github.com/brandon-level/bbcf8d4a35beb1887b063ab815c91b02

Generation Details

java -jar openapi-generator/modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g kotlin -i x-enum-varnames-demo.yml -o ~/Desktop/openapi-demo-out -p library=multiplatform --additional-properties dateLibrary=kotlinx-datetime

Steps to reproduce

Generate classes from the schema, compare generated classes

Suggest a fix

Update code generation for inline enums using x-enum-varnames to override toString(), like for -level enum declarations.