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

llama_stack/providers/remote/inference/databricks/config.py

@@ -14,12 +14,12 @@ from llama_stack.schema_utils import json_schema_type
 
 @json_schema_type
 class DatabricksImplConfig(RemoteInferenceProviderConfig):
-    url: str = Field(
+    url: str | None = Field(
         default=None,
         description="The URL for the Databricks model serving endpoint",
     )
     api_token: SecretStr = Field(
-        default=SecretStr(None),
+        default=SecretStr(None),  # type: ignore[arg-type]
         description="The Databricks API token",
     )