validateEveryToJSON ignores new response handlers

We’ve implemented a new handler (for streaming responses) by providing instances of `HasServer`, `HasClient`, `Accept`, and `HasSwagger`. This works reasonably well, but now we've run into an issue where the type-level machinery used by `validateEveryToJSON` (i.e. `BodyTypes`) can’t be extended in a similar way, and in fact it silently ignores endpoints that use the new type. I don’t know if this approach _can_ be made to handle this, but it would be an improvement if it at least gave an error saying that it can’t discover all the types.

The problem is the “default” case on this line:
https://github.com/haskell-servant/servant-swagger/blob/735657be74c1fb701c18d6eeac4d3b2ba23b66e1/src/Servant/Swagger/Internal/TypeLevel/API.hs#L81

Start with src/Servant/Swagger/Internal/TypeLevel/API.hs around the default case at line 81, then trace how BodyTypes is used by validateEveryToJSON. Reproduce the behavior with an endpoint using the new response handler and determine how unsupported types should be reported. Done means such endpoints are no longer silently ignored and instead produce a clear error or an agreed limitation.

Written by the indexing model from the issue text.