chore: OpenAIMixin implements ModelsProtocolPrivate (#3662)

# What does this PR do?

add ModelsProtocolPrivate methods to OpenAIMixin

this will allow providers using OpenAIMixin to use a common interface


## Test Plan

ci w/ new tests

llama_stack/providers/utils/inference/openai_mixin.py

@@ -26,14 +26,14 @@ from llama_stack.apis.inference import (
 from llama_stack.apis.models import ModelType
 from llama_stack.core.request_headers import NeedsRequestProviderData
 from llama_stack.log import get_logger
-from llama_stack.providers.utils.inference.model_registry import ModelRegistryHelper
+from llama_stack.providers.datatypes import ModelsProtocolPrivate
 from llama_stack.providers.utils.inference.openai_compat import prepare_openai_completion_params
 from llama_stack.providers.utils.inference.prompt_adapter import localize_image_content
 
 logger = get_logger(name=__name__, category="providers::utils")
 
 
-class OpenAIMixin(ModelRegistryHelper, NeedsRequestProviderData, ABC):
+class OpenAIMixin(ModelsProtocolPrivate, NeedsRequestProviderData, ABC):
     """
     Mixin class that provides OpenAI-specific functionality for inference providers.
     This class handles direct OpenAI API calls using the AsyncOpenAI client.
@@ -73,6 +73,9 @@ class OpenAIMixin(ModelRegistryHelper, NeedsRequestProviderData, ABC):
     # Optional field name in provider data to look for API key, which takes precedence
     provider_data_api_key_field: str | None = None
 
+    # automatically set by the resolver when instantiating the provider
+    __provider_id__: str
+
     @abstractmethod
     def get_api_key(self) -> str:
         """
@@ -356,6 +359,24 @@ class OpenAIMixin(ModelRegistryHelper, NeedsRequestProviderData, ABC):
             usage=usage,
         )
 
+    ###
+    # ModelsProtocolPrivate implementation - provide model management functionality
+    #
+    #  async def register_model(self, model: Model) -> Model: ...
+    #  async def unregister_model(self, model_id: str) -> None: ...
+    #
+    #  async def list_models(self) -> list[Model] | None: ...
+    #  async def should_refresh_models(self) -> bool: ...
+    ##
+
+    async def register_model(self, model: Model) -> Model:
+        if not await self.check_model_availability(model.provider_model_id):
+            raise ValueError(f"Model {model.provider_model_id} is not available from provider {self.__provider_id__}")
+        return model
+
+    async def unregister_model(self, model_id: str) -> None:
+        return None
+
     async def list_models(self) -> list[Model] | None:
         """
         List available models from the provider's /v1/models endpoint augmented with static embedding model metadata.
@@ -400,5 +421,7 @@ class OpenAIMixin(ModelRegistryHelper, NeedsRequestProviderData, ABC):
         """
         if not self._model_cache:
             await self.list_models()
-
         return model in self._model_cache
+
+    async def should_refresh_models(self) -> bool:
+        return False