[R][Client] catch enum classes in toJSON

[joXemMx]

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Related to https://github.com/OpenAPITools/openapi-generator/pull/18183

If an enum-class is parameter of another class, it currently gets serialized by toJSON like every other class:

EnumObject[["enum"]] <-
  self$`enum`$toJSON()

This runs into issues as an enum class is essentially just a class holding a singular string value "ENUM". In the JSON that is being put into the http request, "ENUM" is then turned into "\"ENUM\"", which will not be recognized by the API as it is not a valid enum, but rather the same enum with quotations marks around it.

To circumvent this, I implemented a check in toJSON that looks at the class in JSON form and checks if it has no keys and its only value is a string (this should apply only to enum classes in my mind, but please prove me wrong otherwise).

if (length(names(self$`{{name}}`$toJSON())) == 0L && is.character(jsonlite::fromJSON(self$`{{name}}`$toJSON()))) {
  jsonlite::fromJSON(self$`{{name}}`$toJSON())
} else {
  self$`{{name}}`$toJSON()
}

This way, the enum will be assigned as a string instead of an JSON object and serialized correctly. If you see any better fixes for this problem, please feel free to pitch some ideas. This was just the easiest and most obvious fix for me right now. Technical committee @Ramanth @saigiridhar21

[joXemMx]

This seems to still have some hickups, I converted to draft until I sorted them out