[akka-http] form-urlencoded requestBody with format:binary generates uncompilable code

Presumably in the case of both `UrlencodedFormData` and `MultipartFormData`, optional `format: binary` fields should be excluded from the `UrlencodedFormData` path parameter parsing, emitting a static `None` so it's compatible with the other `Unmarshaller` types.

In the case of a _required_ `format: binary`, we should just emit a user error and fail to generate bad code.

Start by tracing the akka-http generator paths for UrlencodedFormData and MultipartFormData, focusing on how optional and required format: binary fields are parsed. Confirm the generated code for optional fields remains compatible with the other Unmarshaller types. Done means optional binary fields emit static None and required binary fields produce a user error rather than uncompilable code.

Written by the indexing model from the issue text.