GET default CAR response for non-existing file (format=car)

Implementation	bifrost-gateway	boxo	kubo
Conformance Version	main	main	main
Total	❌16	✅7	✅7
Body	❌1	✅1	✅1
Header Accept-Ranges	✅1	✅1	✅1
Header Content-Disposition	✅1	✅1	✅1
Header Content-Type	✅1	✅1	✅1
Header Etag	✅1	✅1	✅1
Header X-Content-Type-Options	✅1	✅1	✅1
Status code	✅1	✅1	✅1