[TCK] - Mising test coverage for the `@ExternalDocumentation` annotation usage

[fabiobrz]

We recently found an issue in the SmallRye OpenAPI implementation, which reveals missing test coverage for the @ExternalDocumentation annotation usage in the MicroProfile OpenAPI TCKs.

When such annotation is used at class or method level, the generated OpenAPI document does not contain the expected externalDocs property.