Kill "remote" providers and fix testing with a remote stack properly (#435)

# What does this PR do?

This PR kills the notion of "pure passthrough" remote providers. You
cannot specify a single provider you must specify a whole distribution
(stack) as remote.

This PR also significantly fixes / upgrades testing infrastructure so
you can now test against a remotely hosted stack server by just doing

```bash
pytest -s -v -m remote  test_agents.py \
  --inference-model=Llama3.1-8B-Instruct --safety-shield=Llama-Guard-3-1B \
  --env REMOTE_STACK_URL=http://localhost:5001
```

Also fixed `test_agents_persistence.py` (which was broken) and killed
some deprecated testing functions.

## Test Plan

All the tests.

llama_stack/providers/tests/inference/test_text_inference.py

@@ -147,9 +147,9 @@ class TestInference:
 
         user_input = "Michael Jordan was born in 1963. He played basketball for the Chicago Bulls. He retired in 2003."
         response = await inference_impl.completion(
+            model_id=inference_model,
             content=user_input,
             stream=False,
-            model=inference_model,
             sampling_params=SamplingParams(
                 max_tokens=50,
             ),