A fit in the URL with an empty value (fit=) was treated as missing, so
it was served as cover and verified against a signature made for cover.
It is now a 400 naming fit, the same rule the route applies to an empty
q. It is checked before the existing fit-mode check, which takes an
empty fit as missing; any other value still goes through that check
unchanged. Only a fit missing from the URL is cover.
Model: opus-5-5
A fit in the URL with an empty value is served as cover today and
verifies against a signature made for cover. This test asks for a 400
naming fit instead; it fails until the route refuses it.
Model: opus-5-5