feat(registry): more flexible model lookup (#2859)

This PR updates model registration and lookup behavior to be slightly
more general / flexible. See
https://github.com/meta-llama/llama-stack/issues/2843 for more details.

Note that this change is backwards compatible given the design of the
`lookup_model()` method.

## Test Plan

Added unit tests

llama_stack/distribution/routing_tables/models.py

@@ -13,7 +13,7 @@ from llama_stack.distribution.datatypes import (
 )
 from llama_stack.log import get_logger
 
-from .common import CommonRoutingTableImpl
+from .common import CommonRoutingTableImpl, lookup_model
 
 logger = get_logger(name=__name__, category="core")
 
@@ -36,10 +36,11 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
         return OpenAIListModelsResponse(data=openai_models)
 
     async def get_model(self, model_id: str) -> Model:
-        model = await self.get_object_by_identifier("model", model_id)
-        if model is None:
-            raise ValueError(f"Model '{model_id}' not found")
-        return model
+        return await lookup_model(self, model_id)
+
+    async def get_provider_impl(self, model_id: str) -> Any:
+        model = await lookup_model(self, model_id)
+        return self.impls_by_provider_id[model.provider_id]
 
     async def register_model(
         self,
@@ -49,24 +50,33 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
         metadata: dict[str, Any] | None = None,
         model_type: ModelType | None = None,
     ) -> Model:
-        if provider_model_id is None:
-            provider_model_id = model_id
         if provider_id is None:
             # If provider_id not specified, use the only provider if it supports this model
             if len(self.impls_by_provider_id) == 1:
                 provider_id = list(self.impls_by_provider_id.keys())[0]
             else:
                 raise ValueError(
-                    f"No provider specified and multiple providers available. Please specify a provider_id. Available providers: {self.impls_by_provider_id.keys()}"
+                    f"Please specify a provider_id for model {model_id} since multiple providers are available: {self.impls_by_provider_id.keys()}.\n\n"
+                    "Use the provider_id as a prefix to disambiguate, e.g. 'provider_id/model_id'."
                 )
-        if metadata is None:
-            metadata = {}
-        if model_type is None:
-            model_type = ModelType.llm
+
+        provider_model_id = provider_model_id or model_id
+        metadata = metadata or {}
+        model_type = model_type or ModelType.llm
         if "embedding_dimension" not in metadata and model_type == ModelType.embedding:
             raise ValueError("Embedding model must have an embedding dimension in its metadata")
+
+        # an identifier different than provider_model_id implies it is an alias, so that
+        # becomes the globally unique identifier. otherwise provider_model_ids can conflict,
+        # so as a general rule we must use the provider_id to disambiguate.
+
+        if model_id != provider_model_id:
+            identifier = model_id
+        else:
+            identifier = f"{provider_id}/{provider_model_id}"
+
         model = ModelWithOwner(
-            identifier=model_id,
+            identifier=identifier,
             provider_resource_id=provider_model_id,
             provider_id=provider_id,
             metadata=metadata,