data-api: drop unused jwks_uri from discovery struct

The CLI never fetches JWKS — that's a server-side verification concern — so modelling the field only created a name/shape coupling to the server. entire.io#2281 renames it to jwks_uris (plural); rather than chase that, drop the field entirely. Go ignores unknown JSON fields on decode, so the server can evolve it freely. Test body now sends jwks_uris to prove we tolerate it.