I haven't seen it mentioned in the docs so far, but is there support for defining the interface using swagger/openapi? Given how tied into MediaWiki the server will be it seems unlikely, but I figured it was worth asking in case it was on a roadmap somewhere.
Not for the Action API or the MediaWiki REST API. The closest current plan I'm aware of is a suggestion to generate a swagger/openapi spec from the MediaWiki REST API (for use by clients that can do something useful with such a spec), but that's kind of the opposite of what you asked about.
RestBase/RestRouter does use swagger, I think.