fix!: support $ref from endpoint response to components/responses

[eli-bl]

Fixes #1125.

This makes it so the following—

components:
  responses:
    GoodResponse:
      description: OK
      content:
        "application/json":
          schema:
            $ref: "#/components/schemas/MyObject"

paths:
  "/endpoint":
    get:
      responses:
        "200":
          $ref: "#/components/responses/GoodResponse"

—is exactly equivalent to:

paths:
  "/endpoint":
    get:
      responses:
        "200":
          description: OK
          content:
            "application/json":
              schema:
                $ref: "#/components/schemas/MyObject"

Previously, it would have failed to follow the response reference and behaved as if the response had no content.

This still requires the response to be fully defined inline in components/responses; it can't itself consist of just a $ref.

[dbanty]

Looks good to me! Will include as a breaking change, since documents which previously generated (even though the responses would be empty) may stop generating.

[eli-bl]

@dbanty:

Looks good to me! Will include as a breaking change, since documents which previously generated (even though the responses would be empty) may stop generating.

You mean they would stop generating if there was an invalid response in the spec? Is it always the policy for this project that "it used to incorrectly pass when you used an invalid spec (even though the generated code wouldn't actually work), now it errors out" would constitute a breaking change rather than a bugfix or feature addition?

[eli-bl]

(Sorry, deleted the branch by total stupid accident)

[squat]

@eli-bl any chance you would re-open this PR to try merging again? The lack of this basic OpenAPI functionality is a big gap in the project.

[eli-bl]

@squat Thanks for reminding me - I had closed this by accident earlier, then I saw that I needed to resolve conflicts due to other changes in the meantime, now I've resolved them.