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/fixtures.py

@@ -21,7 +21,7 @@ from llama_stack.providers.remote.inference.fireworks import FireworksImplConfig
 from llama_stack.providers.remote.inference.ollama import OllamaImplConfig
 from llama_stack.providers.remote.inference.together import TogetherImplConfig
 from llama_stack.providers.remote.inference.vllm import VLLMInferenceAdapterConfig
-from llama_stack.providers.tests.resolver import resolve_impls_for_test_v2
+from llama_stack.providers.tests.resolver import construct_stack_for_test
 
 from ..conftest import ProviderFixture, remote_stack_fixture
 from ..env import get_env_or_fail
@@ -182,15 +182,11 @@ INFERENCE_FIXTURES = [
 async def inference_stack(request, inference_model):
     fixture_name = request.param
     inference_fixture = request.getfixturevalue(f"inference_{fixture_name}")
-    impls = await resolve_impls_for_test_v2(
+    test_stack = await construct_stack_for_test(
         [Api.inference],
         {"inference": inference_fixture.providers},
         inference_fixture.provider_data,
-        models=[
-            ModelInput(
-                model_id=inference_model,
-            )
-        ],
+        models=[ModelInput(model_id=inference_model)],
     )
 
-    return (impls[Api.inference], impls[Api.models])
+    return test_stack.impls[Api.inference], test_stack.impls[Api.models]

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,
             ),

llama_stack/providers/tests/inference/test_vision_inference.py

@@ -55,7 +55,7 @@ class TestVisionModelInference:
             )
 
         response = await inference_impl.chat_completion(
-            model=inference_model,
+            model_id=inference_model,
             messages=[
                 UserMessage(content="You are a helpful assistant."),
                 UserMessage(content=[image, "Describe this image in two sentences."]),
@@ -102,7 +102,7 @@ class TestVisionModelInference:
             response = [
                 r
                 async for r in await inference_impl.chat_completion(
-                    model=inference_model,
+                    model_id=inference_model,
                     messages=[
                         UserMessage(content="You are a helpful assistant."),
                         UserMessage(