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/distribution/client.py

@@ -20,21 +20,17 @@ from llama_stack.providers.datatypes import RemoteProviderConfig
 _CLIENT_CLASSES = {}
 
 
-async def get_client_impl(
-    protocol, additional_protocol, config: RemoteProviderConfig, _deps: Any
-):
-    client_class = create_api_client_class(protocol, additional_protocol)
+async def get_client_impl(protocol, config: RemoteProviderConfig, _deps: Any):
+    client_class = create_api_client_class(protocol)
     impl = client_class(config.url)
     await impl.initialize()
     return impl
 
 
-def create_api_client_class(protocol, additional_protocol) -> Type:
+def create_api_client_class(protocol) -> Type:
     if protocol in _CLIENT_CLASSES:
         return _CLIENT_CLASSES[protocol]
 
-    protocols = [protocol, additional_protocol] if additional_protocol else [protocol]
-
     class APIClient:
         def __init__(self, base_url: str):
             print(f"({protocol.__name__}) Connecting to {base_url}")
@@ -42,11 +38,10 @@ def create_api_client_class(protocol, additional_protocol) -> Type:
             self.routes = {}
 
             # Store routes for this protocol
-            for p in protocols:
-                for name, method in inspect.getmembers(p):
-                    if hasattr(method, "__webmethod__"):
-                        sig = inspect.signature(method)
-                        self.routes[name] = (method.__webmethod__, sig)
+            for name, method in inspect.getmembers(protocol):
+                if hasattr(method, "__webmethod__"):
+                    sig = inspect.signature(method)
+                    self.routes[name] = (method.__webmethod__, sig)
 
         async def initialize(self):
             pass
@@ -160,17 +155,16 @@ def create_api_client_class(protocol, additional_protocol) -> Type:
             return ret
 
     # Add protocol methods to the wrapper
-    for p in protocols:
-        for name, method in inspect.getmembers(p):
-            if hasattr(method, "__webmethod__"):
+    for name, method in inspect.getmembers(protocol):
+        if hasattr(method, "__webmethod__"):
 
-                async def method_impl(self, *args, method_name=name, **kwargs):
-                    return await self.__acall__(method_name, *args, **kwargs)
+            async def method_impl(self, *args, method_name=name, **kwargs):
+                return await self.__acall__(method_name, *args, **kwargs)
 
-                method_impl.__name__ = name
-                method_impl.__qualname__ = f"APIClient.{name}"
-                method_impl.__signature__ = inspect.signature(method)
-                setattr(APIClient, name, method_impl)
+            method_impl.__name__ = name
+            method_impl.__qualname__ = f"APIClient.{name}"
+            method_impl.__signature__ = inspect.signature(method)
+            setattr(APIClient, name, method_impl)
 
     # Name the class after the protocol
     APIClient.__name__ = f"{protocol.__name__}Client"