chore: create OpenAIMixin for inference providers with an OpenAI-compat API that need to implement openai_* methods (#2835)

# What does this PR do?

add an `OpenAIMixin` for use by inference providers who remote endpoints
support an OpenAI compatible API.

use is demonstrated by refactoring
- OpenAIInferenceAdapter
- NVIDIAInferenceAdapter (adds embedding support)
- LlamaCompatInferenceAdapter

## Test Plan

existing unit and integration tests

llama_stack/providers/utils/inference/model_registry.py

@@ -10,12 +10,15 @@ from pydantic import BaseModel, Field
 
 from llama_stack.apis.common.errors import UnsupportedModelError
 from llama_stack.apis.models import ModelType
+from llama_stack.log import get_logger
 from llama_stack.models.llama.sku_list import all_registered_models
 from llama_stack.providers.datatypes import Model, ModelsProtocolPrivate
 from llama_stack.providers.utils.inference import (
     ALL_HUGGINGFACE_REPOS_TO_MODEL_DESCRIPTOR,
 )
 
+logger = get_logger(name=__name__, category="core")
+
 
 # TODO: this class is more confusing than useful right now. We need to make it
 # more closer to the Model class.
@@ -98,6 +101,9 @@ class ModelRegistryHelper(ModelsProtocolPrivate):
         :param model: The model identifier to check.
         :return: True if the model is available dynamically, False otherwise.
         """
+        logger.info(
+            f"check_model_availability is not implemented for {self.__class__.__name__}. Returning False by default."
+        )
         return False
 
     async def register_model(self, model: Model) -> Model: