fix(types): address type-safety issues from mypy PR review

- CreateBatchRequest.output_expires_after: drop Optional since total=False
  already makes the key absent-or-present; Optional[T] incorrectly allowed
  the key to exist with value None, which is incompatible with the OpenAI
  SDK's OutputExpiresAfter | NotGiven expectation on batches.create()
- cost_tracking_settings._resolve_model_for_cost_lookup: replace implicit
  object-to-str returns with explicit str() calls so the function is safe
  even if the surrounding truthiness guards are later weakened

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

litellm/proxy/management_endpoints/cost_tracking_settings.py

@@ -67,7 +67,7 @@ def _resolve_model_for_cost_lookup(model: str) -> Tuple[str, Optional[str]]:
                         f"Resolved model '{model}' to base_model '{base_model}' from router"
                     )
                     custom_llm_provider = litellm_params.get("custom_llm_provider")
-                    return base_model, custom_llm_provider
+                    return str(base_model), str(custom_llm_provider) if custom_llm_provider is not None else None
 
                 resolved_model = litellm_params.get("model")
 
@@ -76,7 +76,7 @@ def _resolve_model_for_cost_lookup(model: str) -> Tuple[str, Optional[str]]:
                         f"Resolved model '{model}' to '{resolved_model}' from router"
                     )
                     custom_llm_provider = litellm_params.get("custom_llm_provider")
-                    return resolved_model, custom_llm_provider
+                    return str(resolved_model), str(custom_llm_provider) if custom_llm_provider is not None else None
         except Exception as e:
             verbose_proxy_logger.debug(
                 f"Could not resolve model '{model}' from router: {e}"

litellm/types/llms/openai.py

@@ -424,7 +424,7 @@ class CreateBatchRequest(TypedDict, total=False):
     endpoint: Literal["/v1/chat/completions", "/v1/embeddings", "/v1/completions"]
     input_file_id: str
     metadata: Optional[Dict[str, str]]
-    output_expires_after: Optional[FileExpiresAfter]
+    output_expires_after: FileExpiresAfter
     extra_headers: Optional[Dict[str, str]]
     extra_body: Optional[Dict[str, str]]
     timeout: Optional[float]