Fix missing @Serializable on enums for kotlin/jvm-retrofit2

[yzaoui]

This adds the missing @Serializable annotation when using the kotlin generator with serializationLibrary=kotlinx_serialization and library=jvm-retrofit2.
Currently, the generated enum classes add @SerialName to each child of the enum, but these annotations do not work because @Serializable is missing from the enum class. This can be seen in the docs here. As a result, currently a setup like this to control ascending/descending sort order

enum class SortOrder(val value: String) {
    @SerialName(value = "-name") _NAME("-name"),
    @SerialName(value = "name") NAME("name"),
}

is serialized as "_NAME" and "NAME", rather than the expected "-name" and "name". Adding the missing annotation fixes this.

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[yzaoui]

Could @4brunu take a look?

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/17506211302/job/49735197450?pr=21907

please follow step 3 to update the samples

[wing328]

thanks for the fix, which has been merged

have a nice weekend

[yzaoui]

Thank you William!