feat(registry): make the Stack query providers for model listing (#2862)

This flips #2823 and #2805 by making the Stack periodically query the
providers for models rather than the providers going behind the back and
calling "register" on to the registry themselves. This also adds support
for model listing for all other providers via `ModelRegistryHelper`.
Once this is done, we do not need to manually list or register models
via `run.yaml` and it will remove both noise and annoyance (setting
`INFERENCE_MODEL` environment variables, for example) from the new user
experience.

In addition, it adds a configuration variable `allowed_models` which can
be used to optionally restrict the set of models exposed from a
provider.

llama_stack/distribution/routing_tables/models.py

@@ -10,6 +10,7 @@ from typing import Any
 from llama_stack.apis.models import ListModelsResponse, Model, Models, ModelType, OpenAIListModelsResponse, OpenAIModel
 from llama_stack.distribution.datatypes import (
     ModelWithOwner,
+    RegistryEntrySource,
 )
 from llama_stack.log import get_logger
 
@@ -19,6 +20,26 @@ logger = get_logger(name=__name__, category="core")
 
 
 class ModelsRoutingTable(CommonRoutingTableImpl, Models):
+    listed_providers: set[str] = set()
+
+    async def refresh(self) -> None:
+        for provider_id, provider in self.impls_by_provider_id.items():
+            refresh = await provider.should_refresh_models()
+            if not (refresh or provider_id in self.listed_providers):
+                continue
+
+            try:
+                models = await provider.list_models()
+            except Exception as e:
+                logger.exception(f"Model refresh failed for provider {provider_id}: {e}")
+                continue
+
+            self.listed_providers.add(provider_id)
+            if models is None:
+                continue
+
+            await self.update_registered_models(provider_id, models)
+
     async def list_models(self) -> ListModelsResponse:
         return ListModelsResponse(data=await self.get_all_with_type("model"))
 
@@ -81,6 +102,7 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
             provider_id=provider_id,
             metadata=metadata,
             model_type=model_type,
+            source=RegistryEntrySource.via_register_api,
         )
         registered_model = await self.register_object(model)
         return registered_model
@@ -91,7 +113,7 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
             raise ValueError(f"Model {model_id} not found")
         await self.unregister_object(existing_model)
 
-    async def update_registered_llm_models(
+    async def update_registered_models(
         self,
         provider_id: str,
         models: list[Model],
@@ -102,18 +124,19 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
         # from run.yaml) that we need to keep track of
         model_ids = {}
         for model in existing_models:
-            # we leave embeddings models alone because often we don't get metadata
-            # (embedding dimension, etc.) from the provider
-            if model.provider_id == provider_id and model.model_type == ModelType.llm:
+            if model.provider_id != provider_id:
+                continue
+            if model.source == RegistryEntrySource.via_register_api:
                 model_ids[model.provider_resource_id] = model.identifier
-                logger.debug(f"unregistering model {model.identifier}")
-                await self.unregister_object(model)
+                continue
+
+            logger.debug(f"unregistering model {model.identifier}")
+            await self.unregister_object(model)
 
         for model in models:
-            if model.model_type != ModelType.llm:
-                continue
             if model.provider_resource_id in model_ids:
-                model.identifier = model_ids[model.provider_resource_id]
+                # avoid overwriting a non-provider-registered model entry
+                continue
 
             logger.debug(f"registering model {model.identifier} ({model.provider_resource_id})")
             await self.register_object(
@@ -123,5 +146,6 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
                     provider_id=provider_id,
                     metadata=model.metadata,
                     model_type=model.model_type,
+                    source=RegistryEntrySource.listed_from_provider,
                 )
             )