[BUG] OpenAPI callbacks on operations doesn't match spec

[seglo]

Tapir version: 1.0.0-M5

Scala version: 2.12.12

Describe the bug

In tapir's Open API object model there's support to add callbacks to Operations and Components. Both models have an addComponents helper that accepts a Callback. When I define a Callback and render the docs as YAML I'll get something like the following:

openapi: 3.0.3
...
paths:
  /my_endpoint:
    post:
      ...
      callbacks:
        myCallback:
          pathItems:
            /my_callback:
              put:
                ...

But based on the OpenAPI spec for callbacks the pathItems level isn't valid. If I omit this I get the expected result and I can render callbacks correctly with the tapir redoc library.

paths:
  /my_endpoint:
    post:
      ...
      callbacks:
        myCallback:
          /my_callback:
            put:
                ...

How to reproduce?

I have some hacky code that uses a "callback" endpoint that I defined with the endpoint DSL in the Callback added to an existing endpoint operation my_endpoint:

  def customizeDocs(docs: OpenAPI): OpenAPI = {
    import com.softwaremill.quicklens._
    val callback = Callback(ListMap("/my_callback" -> docs.paths.pathItems("/my_callback")))
    docs
      .modify(_.paths.pathItems.at("/my_endpoint").post.each).using(_.addCallback("myCallback", callback))
  }

I think the issue might be with the context of where a Callback is added. Callback accepts a list of map of PathItem, but the pathItem stanza is rendered into the API docs when it shouldn't.

I could probably PR a fix, but I'm hesitant because I don't know why Callback is the way it is and I don't want to introduce a regression for anyone.