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/conftest.py

@@ -35,8 +35,8 @@ def remote_stack_fixture() -> ProviderFixture:
     return ProviderFixture(
         providers=[
             Provider(
-                provider_id="remote",
-                provider_type="remote",
+                provider_id="test::remote",
+                provider_type="test::remote",
                 config=config.model_dump(),
             )
         ],