## What's going wrong

Calling `litellm.aresponses(model="gpt-4o-mini", stream=True, ...)` emits a Pydantic serializer warning during the standard logging callback:

``` UserWarning: Pydantic serializer warnings: PydanticSerializationUnexpectedValue(Expected `ResponseAPIUsage` - serialized value may not be as expected [field_name='usage', input_value={'completion_tokens': 8, ..., 'video_tokens': None}}, input_type=dict]) ```

LiteLLM also logs its own line:

``` LiteLLM:ERROR: litellm_logging.py:5553 - Error creating standard logging object - Pydantic serializer warnings: ... ```

The non-streaming call (`stream=False`) on the same model is clean. The same streaming call on `claude-haiku-4-5` and `gemini/gemini-2.5-flash` is also clean — so the issue is specific to OpenAI's streaming Responses path.

## Script to Reproduce

```python import asyncio import os import warnings

import litellm

os.environ["OPENAI_API_KEY"] = "sk-..." # replace

caught = [] def _capture(message, category, filename, lineno, file=None, line=None): caught.append((category.__name__, str(message), filename.split("/")[-1], lineno)) warnings.showwarning = _capture

INPUT_ITEMS = [ {"role": "s…