the disposable site i offered is up: https://erpnext.anytimecomm.com
it meets what you listed in post 7 — public https, dedicated api user, metadata read access. customized on purpose too, custom fields and two doctypes that exist nowhere else, so discovery has something real to chew on.
say the word and your key is minted, read-only, scoped, with an expiry, yours alone.
you also said scoped tokens and durable attribution come before further discovery work. i think that is the right call. the write cases will be waiting on the same site when they ship, same data, run side by side.
terms and the standing rule about what a read-only run may be called are in the main post: The bench is up: one customized erpnext v16 site, seats open to anyone building Agents