Mixing OpenAPI 3.0.x and 3.1.x

<img width="1305" height="222" alt="Image" src="https://github.com/user-attachments/assets/19763cd5-4d4d-440e-b60c-0faa4102aca7" />

most issues are coming from
`nullable:true`.
it should be
```
     anyOf:
        - type: ...
        - type: 'null'
```