[BUG][GO] Compilation error, undefined OneOf

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?
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Description

Generated code doesn't compile due to undefined OneOf.

openapi-generator version

7.0.0 and 7.0.1

OpenAPI declaration file content or url

https://gist.github.com/morus12/d99446557841985e4912a5453422956d

Generation Details

openapi-generator generate -i spec.yaml -o client -g go

Steps to reproduce

1. Generate client
2. Run go test

./api_default.go:28:8: undefined: OneOf
./api_default.go:31:38: undefined: OneOf

Related issues/PRs

Suggest a fix

Start with the OpenAPI declaration linked in the issue and reproduce it using openapi-generator generate -i spec.yaml -o client -g go. Inspect the generated api_default.go around the reported OneOf references, then run go test; done means the generated client compiles without undefined OneOf errors.