chore: turn OpenAIMixin into a pydantic.BaseModel (#3671)

# What does this PR do?

- implement get_api_key instead of relying on
LiteLLMOpenAIMixin.get_api_key
 - remove use of LiteLLMOpenAIMixin
 - add default initialize/shutdown methods to OpenAIMixin
 - remove __init__s to allow proper pydantic construction
- remove dead code from vllm adapter and associated / duplicate unit
tests
 - update vllm adapter to use openaimixin for model registration
 - remove ModelRegistryHelper from fireworks & together adapters
 - remove Inference from nvidia adapter
 - complete type hints on embedding_model_metadata
- allow extra fields on OpenAIMixin, for model_store, __provider_id__,
etc
 - new recordings for ollama
 - enhance the list models error handling
- update cerebras (remove cerebras-cloud-sdk) and anthropic (custom
model listing) inference adapters
 - parametrized test_inference_client_caching
- remove cerebras, databricks, fireworks, together from blanket mypy
exclude
 - removed unnecessary litellm deps

## Test Plan

ci

pyproject.toml

@@ -278,14 +278,10 @@ exclude = [
     "^llama_stack/providers/remote/datasetio/huggingface/",
     "^llama_stack/providers/remote/datasetio/nvidia/",
     "^llama_stack/providers/remote/inference/bedrock/",
-    "^llama_stack/providers/remote/inference/cerebras/",
-    "^llama_stack/providers/remote/inference/databricks/",
-    "^llama_stack/providers/remote/inference/fireworks/",
     "^llama_stack/providers/remote/inference/nvidia/",
     "^llama_stack/providers/remote/inference/passthrough/",
     "^llama_stack/providers/remote/inference/runpod/",
     "^llama_stack/providers/remote/inference/tgi/",
-    "^llama_stack/providers/remote/inference/together/",
     "^llama_stack/providers/remote/inference/watsonx/",
     "^llama_stack/providers/remote/safety/bedrock/",
     "^llama_stack/providers/remote/safety/nvidia/",