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/test_persistence.py

@@ -0,0 +1,122 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import pytest
+
+from llama_stack.apis.agents import *  # noqa: F403
+from llama_stack.providers.datatypes import *  # noqa: F403
+
+from llama_stack.providers.utils.kvstore import kvstore_impl, SqliteKVStoreConfig
+from .fixtures import pick_inference_model
+
+from .utils import create_agent_session
+
+
+@pytest.fixture
+def sample_messages():
+    return [
+        UserMessage(content="What's the weather like today?"),
+    ]
+
+
+@pytest.fixture
+def common_params(inference_model):
+    inference_model = pick_inference_model(inference_model)
+
+    return dict(
+        model=inference_model,
+        instructions="You are a helpful assistant.",
+        enable_session_persistence=True,
+        sampling_params=SamplingParams(temperature=0.7, top_p=0.95),
+        input_shields=[],
+        output_shields=[],
+        tools=[],
+        max_infer_iters=5,
+    )
+
+
+class TestAgentPersistence:
+    @pytest.mark.asyncio
+    async def test_delete_agents_and_sessions(self, agents_stack, common_params):
+        agents_impl = agents_stack.impls[Api.agents]
+        agent_id, session_id = await create_agent_session(
+            agents_impl,
+            AgentConfig(
+                **{
+                    **common_params,
+                    "input_shields": [],
+                    "output_shields": [],
+                }
+            ),
+        )
+
+        run_config = agents_stack.run_config
+        provider_config = run_config.providers["agents"][0].config
+        persistence_store = await kvstore_impl(
+            SqliteKVStoreConfig(**provider_config["persistence_store"])
+        )
+
+        await agents_impl.delete_agents_session(agent_id, session_id)
+        session_response = await persistence_store.get(
+            f"session:{agent_id}:{session_id}"
+        )
+
+        await agents_impl.delete_agents(agent_id)
+        agent_response = await persistence_store.get(f"agent:{agent_id}")
+
+        assert session_response is None
+        assert agent_response is None
+
+    @pytest.mark.asyncio
+    async def test_get_agent_turns_and_steps(
+        self, agents_stack, sample_messages, common_params
+    ):
+        agents_impl = agents_stack.impls[Api.agents]
+
+        agent_id, session_id = await create_agent_session(
+            agents_impl,
+            AgentConfig(
+                **{
+                    **common_params,
+                    "input_shields": [],
+                    "output_shields": [],
+                }
+            ),
+        )
+
+        # Create and execute a turn
+        turn_request = dict(
+            agent_id=agent_id,
+            session_id=session_id,
+            messages=sample_messages,
+            stream=True,
+        )
+
+        turn_response = [
+            chunk async for chunk in await agents_impl.create_agent_turn(**turn_request)
+        ]
+
+        final_event = turn_response[-1].event.payload
+        turn_id = final_event.turn.turn_id
+
+        provider_config = agents_stack.run_config.providers["agents"][0].config
+        persistence_store = await kvstore_impl(
+            SqliteKVStoreConfig(**provider_config["persistence_store"])
+        )
+        turn = await persistence_store.get(f"session:{agent_id}:{session_id}:{turn_id}")
+        response = await agents_impl.get_agents_turn(agent_id, session_id, turn_id)
+
+        assert isinstance(response, Turn)
+        assert response == final_event.turn
+        assert turn == final_event.turn.model_dump_json()
+
+        steps = final_event.turn.steps
+        step_id = steps[0].step_id
+        step_response = await agents_impl.get_agents_step(
+            agent_id, session_id, turn_id, step_id
+        )
+
+        assert step_response.step == steps[0]