[BUG][erlang-server] Generated code doesn't use schema

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• 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?
• [] [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The generated code doesn't use the schema to validate the body. Instead it generates many different checks.

request_params('AccessTokenRequest') ->
    [
        'grant_type',
        'nfInstanceId',
        'scope',
        'Content-Encoding',
        'Accept-Encoding',
        'nfType',
        'targetNfType',
        'targetNfInstanceId',
        'requesterPlmn',
        'requesterPlmnList',
        'requesterSnssaiList',
        'requesterFqdn',
        'requesterSnpnList',
        'targetPlmn',
        'targetSnssaiList',
        'targetNsiList',
        'targetNfSetId',
        'targetNfServiceSetId',
        'hnrfAccessTokenUri'
    ];

This could have "worked" but the generated code that check in body

get_value(body, _Name, Req0) ->
    {ok, Body, Req} = cowboy_req:read_body(Req0),
    case prepare_body(Body) of
        {error, Reason} ->
            {error, Reason, Req};
        Value ->
            {Value, Req}
    end;

this will only work once since read_body will return <<>> the second time.
This code also decodes jsx:decode once per key in the body it wants to verify.

openapi-generator version

openapi-generator-cli 5.2.0-SNAPSHOT
commit : 24836cd

OpenAPI declaration file content or url

https://raw.githubusercontent.com/jdegre/5GC_APIs/master/TS29510_Nnrf_AccessToken.yaml

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

request_params('AccessTokenRequest') ->
    [
        'AccessTokenReq'
    ];

request_param_info('AccessTokenRequest', 'AccessTokenReq') ->
    #{
        source =>   body,
        rules => [
            schema,
            required
        ]
    };

Start with the generated Erlang entry points request_params/2, request_param_info/2, and get_value/3, using the linked TS29510_Nnrf_AccessToken.yaml declaration as the reproduction input. Trace body handling and schema validation, then verify that generated code applies the schema and does not reread or decode the request body once per key.