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/agents/fixtures.py

@@ -16,10 +16,9 @@ from llama_stack.providers.inline.agents.meta_reference import (
     MetaReferenceAgentsImplConfig,
 )
 
-from llama_stack.providers.tests.resolver import resolve_impls_for_test_v2
+from llama_stack.providers.tests.resolver import construct_stack_for_test
 from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
 from ..conftest import ProviderFixture, remote_stack_fixture
-from ..safety.fixtures import get_shield_to_register
 
 
 def pick_inference_model(inference_model):
@@ -60,7 +59,7 @@ AGENTS_FIXTURES = ["meta_reference", "remote"]
 
 
 @pytest_asyncio.fixture(scope="session")
-async def agents_stack(request, inference_model, safety_model):
+async def agents_stack(request, inference_model, safety_shield):
     fixture_dict = request.param
 
     providers = {}
@@ -71,13 +70,10 @@ async def agents_stack(request, inference_model, safety_model):
         if fixture.provider_data:
             provider_data.update(fixture.provider_data)
 
-    shield_input = get_shield_to_register(
-        providers["safety"][0].provider_type, safety_model
-    )
     inference_models = (
         inference_model if isinstance(inference_model, list) else [inference_model]
     )
-    impls = await resolve_impls_for_test_v2(
+    test_stack = await construct_stack_for_test(
         [Api.agents, Api.inference, Api.safety, Api.memory],
         providers,
         provider_data,
@@ -87,6 +83,6 @@ async def agents_stack(request, inference_model, safety_model):
             )
             for model in inference_models
         ],
-        shields=[shield_input],
+        shields=[safety_shield],
     )
-    return impls[Api.agents], impls[Api.memory]
+    return test_stack