`4XX` status codes aren't understood like `400`, `401`, etc.

[Monkatraz]

This is fairly simple:

# this gets understood correctly
responses:
  200: { description: "OK" }
  400: { description: "Bad Syntax" }

# this doesn't
responses:
  200: { description: "OK" }
  4XX: { description: "Error" }
  
# (imagine that the 400/4XX codes returned a schema named `ApiError`)

The return types of functions will, example, should be like this:

ApiResponse<void>

But instead, 4XX will be understood as a "success", so it'll actually generate a type like this:

ApiResponse<ApiError>
// or, if the function should return something:
ApiResponse<SomeReturnData | ApiError>