Remove 'some' from factory return type

[lzell]

When trying to set the service as a member on a type, you can hit the compilation error:

Property definition has inferred type 'some OpenAIService', involving the 'some' return type of another declaration

To work around the compilation error, I could specify let service: some OpenAIService = .... A better fix, though, is to remove the some keyword on the factory methods. We don't need some here, because the protocol that our concrete types are conforming to is not a PAT. We don't need to use 'reverse generics' and can instead just rely on vanilla polymorphism through protocols.