You could cite them in the acknowledgements.

Thanks to Foo et al. (2042) for their toolbox which helped the authors to study migratory patterns in unicorns.

I don't think it would be an insult to the intelligence of anybody. Although papers are not a chronological description of everything you tried, if something helped you but did not make it into the final paper, it can go in the acknowledgements.

Adding an item to the bibliography without a corresponding citation is something I have not seen before and it would not surprise me if many jounals would have policies against it.

You could cite them in the acknowledgements.

Thanks to Foo et al. (2042) for their toolbox which helped the authors to study migratory patterns in unicorns.

I don't think it would be an insult to the intelligence of anybody. Although papers are not a chronological description of everything you tried, if something helped you but did not make it into the final paper, it can go in the acknowledgements.

Adding an item to the bibliography without a corresponding citation is something I have not seen before (nor have I checked) and it would not surprise me if many journals would have policies against it. At least one family of journals I know has backreferences, i.e. links from bibliography entries to the places in the main text where the citation occurs, so in this case it would be immediately obvious.