test(testing): add test to detect duplicate endpoint names

[ThijsBroersen]

From my interpretation on the OpenAPI spec, operationId's (endpoint names), should be unique. This test shows it is currently not detected in the EndpointVerifier and when generating the OpenAPI spec it does not warn or fail on duplicate operationId's, it just generates it.

Shall I continue implement the test in the EndpointVerifier?

Should the OpenAPIDocsInterpreter throw an exception on duplicate names? Or perhaps add a toVerifiedOpenAPI: Either[OpenAPIError, OpenAPI] method to the interpreter and not allow e.g. duplicate operationId's?

Refs: https://softwaremill.community/t/duplicate-operationids/416/7

[adamw]

It's a good question on the error modes, there's one place current during the openapi interpreter where we throw exceptions. Not perfect, but introducing Eithers at this point might be confusing. So maybe we should indeed just perform the check in the interpreter itself.