Many of your clients implements a test clients to call TestRail api as part of their pipeline. And these clients must be tested in some way.
In our company (Playtika) we use Wiremock stubs for endpoints are called from pipeline. These stubs correspond to TestRail api documentation (http://docs.gurock.com/testrail-api2/start). But it’s not the best way for testing, because you can change something in the api and we will now about the issue only after crashing in production
What about an implementing a public storage for consumer driven contracts (https://martinfowler.com/articles/consumerDrivenContracts.html) which would be used by the customers interested in it?