session.authFetch.bind(session) needed when passing as argument

Otherwise `this.isActive()` and possibly other method calls with `this` fail.

Many libraries accept authenticated `fetch` as a parameter, for example
https://github.com/rubensworks/rdf-dereference.js?tab=readme-ov-file#input-using-a-custom-fetch-function

I took a quick look at https://github.com/uvdsl/solid-rdf-store
and it looks like it gets `Session` and not just `fetch`