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/safety/test_safety.py

@@ -18,13 +18,6 @@ from llama_stack.distribution.datatypes import *  # noqa: F403
 
 
 class TestSafety:
-    @pytest.mark.asyncio
-    async def test_new_shield(self, safety_stack):
-        _, shields_impl, shield = safety_stack
-        assert shield is not None
-        assert shield.provider_resource_id == shield.identifier
-        assert shield.provider_id is not None
-
     @pytest.mark.asyncio
     async def test_shield_list(self, safety_stack):
         _, shields_impl, _ = safety_stack