I find the ethical boundaries for authors and AI to be pretty simple honestly. Treat it like a human, for these purposes. You wouldn't have to credit someone you spit-balled ideas with or who proofread your work or who gave you some research advice. (Though you should thank them in the acknowledgements. I think you can skip that for a computer program.) OTOH, you would have to credit someone who wrote all or some of your text if you have any artistic integrity at least. Plenty of writers have found artistic uses for GenAI text in their books and disclosed that up front to readers. Why try to hide the use? Unless you know how you're doing it is artistically bankrupt...