API testing during CI/CD

I think you will find interesting to try Qovery - read this article that is talking about isolated environments and feature branching

Full end-to-end API testing can be challenging, since it typically involves a lot of state changes in decentralized components. I have never worked at a place that gets it done right. It is a lot of work to spin up completely isolated test environments for a new build, with all services/microservices, and tear them down again.

If you really want to go that route, the first step would be making sure you're using IAC (infrastructure as code) via Terraform or something similar. Make sure you can say "hey, CI, spin up an entire isolated environment under this domain prefix." Once you can do that, it becomes trivial to do your API tests. Since your current CI build completely controls the state changes on the test environment, you can have an API test client make specific requests and check that the results are valid. Enumerate all of the API endpoints you wish to test.

A much better "bang for buck" setup is to just have excellent unit and functional tests in each service. Where an external service is required, use mock objects. Of course this approach suffers from not being a "real" full test, it is much easier to implement given the resource constraints of a startup. As someone who has built full environment IAC test tooling, it is generally not worth the effort you will sink into solving the problem correctly.