Fix oneOf being sampled twice for the first item in the list

[thiagonuic]

trafficstars

What/Why/How?

Details on this change can be found in the issue: https://github.com/Redocly/openapi-sampler/issues/145

Reference

Testing

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Check yourself

• [x] Code is linted
• [x] Tested
• [x] All new/updated code is covered with tests

Security

• [x] Security impact of change has been considered
• [x] Code follows company security practices and guidelines

[adamaltman]

I think we would want to consider min/max items in the schema definition as well?