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/inline/safety/llama_guard/llama_guard.py

@@ -234,7 +234,7 @@ class LlamaGuardShield:
         # TODO: llama-stack inference protocol has issues with non-streaming inference code
         content = ""
         async for chunk in await self.inference_api.chat_completion(
-            model=self.model,
+            model_id=self.model,
             messages=[shield_input_message],
             stream=True,
         ):