Thanks for the response. You are right. If the third-party API suddenly changes we are in trouble.
Although something as described in
We schedule a build to run the tests of the proxy project in an interval of our liking. For example once a day. The tests for the consumer projects can be run as often as we like. They never call the real API.
or another kind of API Monitoring will serve as an early warning.
In the example above we mostly make sure our side is set up correctly.
If a Google API or some other systems suddenly start to remove fields which were once present we have to find a replacement or call someone in the other department to set things straight again.