bug: MarshalJSON (and UnmarshalJSON) are not generated for the ResponseObject when the response is a oneOf schema about oapi-codegen HOT 4 OPEN

I think this issue is related to #1620

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I found a temporary workaround by using a user template. In the strict-interface template, check if the response's OpenAPI schema is oneOf when the contentType is JSON.

curl https://raw.githubusercontent.com/oapi-codegen/oapi-codegen/v2.3.0/pkg/codegen/templates/strict/strict-interface.tmpl -o strict-interface.tmpl
patch strict-interface.tmpl -o strict-interface.patched.tmpl  < strict-interface.patch

74a75,81
>             {{if eq .NameTagOrContentType "JSON" -}}
>             {{if or (ne 0 (len .Schema.OAPISchema.OneOf)) (ne 0 (len .Schema.OAPISchema.AnyOf)) (ne 0 (len .Schema.OAPISchema.AllOf)) -}}
>             func (response {{$receiverTypeName}}) MarshalJSON() ([]byte, error) {
>                 return response.union.MarshalJSON()
>             }
>             {{end -}}
>             {{end -}}

config.yaml

# yaml-language-server: $schema=https://raw.githubusercontent.com/deepmap/oapi-codegen/HEAD/configuration-schema.json
package: "api"
generate:
  echo-server: true
  models: true
  strict-server: true
  embedded-spec: false
output: api.gen.go
output-options:
  nullable-type: true
  initialism-overrides: true
  user-templates:
    strict/strict-interface.tmpl: ./strict-interface.patched.tmpl

from oapi-codegen.

This issue is also discussed here #970. It comes with a PR #1231 but I'm not sure if it should be merged due to my final comment in the issue

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Also discussed under #970 (comment)
I seems that there're two ways to fix this bug.

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