fix(vllm): repair the return annotation after the PEP 604 migration

litellm_internal_staging moved this module to X | None and dropped
Optional from its typing import, while this branch added a method
annotated Optional["CostResponseTypes"]. The merge was textually clean
but left the module raising NameError on import.

Use the quoted PEP 604 form the rest of the file now uses, and record a
mutable-ok reason for the request_data dict the base class signature
fixes.

litellm/llms/vllm/passthrough/transformation.py

@@ -39,10 +39,10 @@ class VLLMPassthroughConfig(VLLMModelInfo, BasePassthroughConfig):
         model: str,
         custom_llm_provider: str,
         httpx_response: "Response",
-        request_data: dict,
+        request_data: dict,  # mutable-ok: base class signature
         logging_obj: "LiteLLMLoggingObj",
         endpoint: str,
-    ) -> Optional["CostResponseTypes"]:
+    ) -> "CostResponseTypes | None":
         from litellm import encoding
         from litellm.llms.openai.chat.gpt_transformation import OpenAIGPTConfig
         from litellm.types.utils import EmbeddingResponse, ModelResponse, Usage