maint: remove root_path usage in TestClient

[isabelizimm]

With fastapi==0.109.0/ starlette==0.35.0, there have been breaking changes to how root_path is used within APIs. While vetiver doesn't use this directly, it is the way test paths are set inside TestClient, so many of our tests are failing.

from encode/starlette#2400

... if it [root_path] is set, it will be passed to the ASGI apps, that will internally extract/remove it from the paths. But clients communicating with those ASGI apps, if they are indeed mounted at the defined root_path, would have to communicate with it using the root_path prefix. So this PR updates the client to actually require/use that, clients created with root_path would need to be used the same way that clients communicating with those ASGI apps mounted at some prefix path.