[BUG] 5.0 produces invalid Elm code with missing decoders/encoders for custom types

Created by: SiriusStarr

In the petstore example, it looks like custom types's don't have their encoders/decoders properly written:

orderDecoder : Json.Decode.Decoder Order_
orderDecoder =
    Json.Decode.succeed Order_
        |> maybeDecode "id" Json.Decode.int Nothing
        |> maybeDecode "petId" Json.Decode.int Nothing
        |> maybeDecode "quantity" Json.Decode.int Nothing
        |> maybeDecode "shipDate" Api.Time.dateTimeDecoder Nothing
        |> maybeDecode "status" <!!orderStatusDecoder should be here!!> Nothing
        |> maybeDecode "complete" Json.Decode.bool (Just False)

encodeOrderPairs : Order_ -> List EncodedField
encodeOrderPairs model =
    let
        pairs =
            [ maybeEncode "id" Json.Encode.int model.id
            , maybeEncode "petId" Json.Encode.int model.petId
            , maybeEncode "quantity" Json.Encode.int model.quantity
            , maybeEncode "shipDate" Api.Time.encodeDateTime model.shipDate
            , maybeEncode "status" <!!encodeOrderStatus should be here!!> model.status
            , maybeEncode "complete" Json.Encode.bool model.complete
            ]
    in
    pairs

Additionally, there is an erroneous DEcoder in the middle of one of the ENcoders. fixed in master

This is the full diff between the output of 04dfff83 and a manually corrected version:

diff -r elm-openapi-test/src/Api/Data.elm elm-openapi-test-fix/src/Api/Data.elm
202c202
<             , maybeEncode "status"  model.status
---
>             , maybeEncode "status" encodeOrderStatus model.status
246c246
<             , maybeEncode "status"  model.status
---
>             , maybeEncode "status" encodePetStatus model.status
343c343
<         |> maybeDecode "status"  Nothing
---
>         |> maybeDecode "status" orderStatusDecoder Nothing
376c376
<         |> maybeDecode "status"  Nothing
---
>         |> maybeDecode "status" petStatusDecoder Nothing

Originally posted by @SiriusStarr in https://github.com/OpenAPITools/openapi-generator/issues/8218#issuecomment-747686514