It's kinda obvious: have exactly the same handler function with no decorators and really test it like a pure function it is. With an existing opinionated frame

It's kinda obvious: have exactly the same handler function with no decorators and really test it like a pure function it is. With an existing opinionated framework like DRF, just call out to it from your handler, and have exactly one test to ensure that integration keeps working and passes parameters through properly.