fix(Llamacpp): `Llamacpp.url/0` doesn't return the url that was set in config

[alvises]

LLamacpp adapter uses the wrong config field to get the API url. To create a unit test around LLamacpp.url/0 I made the function public. I've also added a nil case in the test setup to be able to run a test without adapter (at the moment tests with Llamacpp adapter are excluded).