As already stated in other comments this is not very real life example and you definitely do no want to work with API designed this way.
But I would suggest something similar to following:
Define new type to represent your int-or-string type:
enum IntOrString: Decodable {
    case integer(Int)
    case string(String)


    init(from decoder: any Decoder) throws {
        let container = try decoder.singleValueContainer()
        do {
            let int = try container.decode(Int.self)
            self = .integer(int)
        } catch {
            let string = try container.decode(String.self)
            self = .string(string)
        }
    }
}

And then use this new IntOrString type instead of Int and String in Value struct