You need to test contracts with external code without having to include full external systems. Unit tests on internal implementation details are fragile as behavior changes. Unit tests on your module's contracts give you confidence in refactoring.

Passing params in instead of making external calls inside your business logic functions can help. DI can help if that's too impractical or unwieldy for whatever reason in the domain.

It's hard to do right the first time - sometimes its fuzzy what's an internal detail vs what's an external contract - but you need to get there ASAP.