feat: add infrastructure to allow inference model discovery

inference providers each have a static list of supported / known models. some also
have access to a dynamic list of currently available models. this change gives
prodivers using the ModelRegistryHelper the ability to combine their static and
dynamic lists.

for instance, OpenAIInferenceAdapter can implement
```
   def query_available_models(self) -> list[str]:
      return [entry.model for entry in self.openai_client.models.list()]
```
to augment its static list w/ a current list from openai.

llama_stack/providers/utils/inference/model_registry.py

@@ -82,9 +82,37 @@ class ModelRegistryHelper(ModelsProtocolPrivate):
     def get_llama_model(self, provider_model_id: str) -> str | None:
         return self.provider_id_to_llama_model_map.get(provider_model_id, None)
 
+    async def query_available_models(self) -> list[str]:
+        """
+        Return a list of available models.
+
+        This is for subclassing purposes, so providers can lookup a list of
+        of currently available models.
+
+        This is combined with the statically configured model entries in
+        `self.alias_to_provider_id_map` to determine which models are
+        available for registration.
+
+        Default implementation returns no models.
+
+        :return: A list of model identifiers (provider_model_ids).
+        """
+        return []
+
     async def register_model(self, model: Model) -> Model:
-        if not (supported_model_id := self.get_provider_model_id(model.provider_resource_id)):
-            raise UnsupportedModelError(model.provider_resource_id, self.alias_to_provider_id_map.keys())
+        # Check if model is supported in static configuration
+        supported_model_id = self.get_provider_model_id(model.provider_resource_id)
+
+        # If not found in static config, check if it's available from provider
+        if not supported_model_id:
+            available_models = await self.query_available_models()
+            if model.provider_resource_id in available_models:
+                supported_model_id = model.provider_resource_id
+            else:
+                # Combine static and dynamic models for error message
+                all_supported_models = list(self.alias_to_provider_id_map.keys()) + available_models
+                raise UnsupportedModelError(model.provider_resource_id, all_supported_models)
+
         provider_resource_id = self.get_provider_model_id(model.model_id)
         if model.model_type == ModelType.embedding:
             # embedding models are always registered by their provider model id and does not need to be mapped to a llama model
@@ -113,6 +141,7 @@ class ModelRegistryHelper(ModelsProtocolPrivate):
                         ALL_HUGGINGFACE_REPOS_TO_MODEL_DESCRIPTOR[llama_model]
                     )
 
+        # Register the model alias, ensuring it maps to the correct provider model id
         self.alias_to_provider_id_map[model.model_id] = supported_model_id
 
         return model

tests/unit/providers/utils/test_model_registry.py

@@ -87,6 +87,37 @@ def helper(known_provider_model: ProviderModelEntry, known_provider_model2: Prov
     return ModelRegistryHelper([known_provider_model, known_provider_model2])
 
 
+class MockModelRegistryHelperWithDynamicModels(ModelRegistryHelper):
+    """Test helper that simulates a provider with dynamically available models."""
+
+    def __init__(self, model_entries: list[ProviderModelEntry], available_models: list[str]):
+        super().__init__(model_entries)
+        self._available_models = available_models
+
+    async def query_available_models(self) -> list[str]:
+        return self._available_models
+
+
+@pytest.fixture
+def dynamic_model() -> Model:
+    """A model that's not in static config but available dynamically."""
+    return Model(
+        provider_id="provider",
+        identifier="dynamic-model",
+        provider_resource_id="dynamic-provider-id",
+    )
+
+
+@pytest.fixture
+def helper_with_dynamic_models(
+    known_provider_model: ProviderModelEntry, known_provider_model2: ProviderModelEntry, dynamic_model: Model
+) -> MockModelRegistryHelperWithDynamicModels:
+    """Helper that includes dynamically available models."""
+    return MockModelRegistryHelperWithDynamicModels(
+        [known_provider_model, known_provider_model2], [dynamic_model.provider_resource_id]
+    )
+
+
 @pytest.mark.asyncio
 async def test_lookup_unknown_model(helper: ModelRegistryHelper, unknown_model: Model) -> None:
     assert helper.get_provider_model_id(unknown_model.model_id) is None
@@ -161,3 +192,66 @@ async def test_unregister_model_during_init(helper: ModelRegistryHelper, known_m
     assert helper.get_provider_model_id(known_model.provider_resource_id) == known_model.provider_model_id
     await helper.unregister_model(known_model.provider_resource_id)
     assert helper.get_provider_model_id(known_model.provider_resource_id) is None
+
+
+@pytest.mark.asyncio
+async def test_register_model_from_query_available_models(
+    helper_with_dynamic_models: MockModelRegistryHelperWithDynamicModels, dynamic_model: Model
+) -> None:
+    """Test that models returned by query_available_models can be registered."""
+    # Verify the model is not in static config
+    assert helper_with_dynamic_models.get_provider_model_id(dynamic_model.provider_resource_id) is None
+
+    # But it should be available via query_available_models
+    available_models = await helper_with_dynamic_models.query_available_models()
+    assert dynamic_model.provider_resource_id in available_models
+
+    # Registration should succeed
+    registered_model = await helper_with_dynamic_models.register_model(dynamic_model)
+    assert registered_model == dynamic_model
+
+    # Model should now be registered and accessible
+    assert (
+        helper_with_dynamic_models.get_provider_model_id(dynamic_model.model_id) == dynamic_model.provider_resource_id
+    )
+
+
+@pytest.mark.asyncio
+async def test_register_model_not_in_static_or_dynamic(
+    helper_with_dynamic_models: MockModelRegistryHelperWithDynamicModels, unknown_model: Model
+) -> None:
+    """Test that models not in static config or dynamic models are rejected."""
+    # Verify the model is not in static config
+    assert helper_with_dynamic_models.get_provider_model_id(unknown_model.provider_resource_id) is None
+
+    # And not in dynamic models
+    available_models = await helper_with_dynamic_models.query_available_models()
+    assert unknown_model.provider_resource_id not in available_models
+
+    # Registration should fail with comprehensive error message
+    with pytest.raises(Exception) as exc_info:  # UnsupportedModelError
+        await helper_with_dynamic_models.register_model(unknown_model)
+
+    # Error should include both static and dynamic models
+    error_str = str(exc_info.value)
+    assert "dynamic-provider-id" in error_str  # dynamic model should be in error
+
+
+@pytest.mark.asyncio
+async def test_register_alias_for_dynamic_model(
+    helper_with_dynamic_models: MockModelRegistryHelperWithDynamicModels, dynamic_model: Model
+) -> None:
+    """Test that we can register an alias that maps to a dynamically available model."""
+    # Create a model with a different identifier but same provider_resource_id
+    alias_model = Model(
+        provider_id=dynamic_model.provider_id,
+        identifier="dynamic-model-alias",
+        provider_resource_id=dynamic_model.provider_resource_id,
+    )
+
+    # Registration should succeed since the provider_resource_id is available dynamically
+    registered_model = await helper_with_dynamic_models.register_model(alias_model)
+    assert registered_model == alias_model
+
+    # Both the original provider_resource_id and the new alias should work
+    assert helper_with_dynamic_models.get_provider_model_id(alias_model.model_id) == dynamic_model.provider_resource_id