fix: convert litellm response with reasoning content to openai message

[zkllll2002]

1. Description

This PR fixes an issue where reasoning content from models accessed via LiteLLM was not being correctly parsed into the ChatCompletionMessage format. This was particularly noticeable when using reasoning models.

2. Context

I am using the openai-agents-python library in my project, and it has been incredibly helpful. Thank you for building such a great tool!

My setup uses litellm to interface with gemini-2.5-pro. I noticed that while the agent could receive a response, the reasoning(thinking) from the Gemini model was lost during the conversion process from the LiteLLM response format to the OpenAI ChatCompletionMessage object.

I saw that PR #871 made progress on a similar issue, but it seems the specific response structure from LiteLLM still requires a small adaptation. This fix adds the necessary logic to ensure that these responses are handled.

Relates to: #871

3. Key Changes

• LitellmConverter.convert_message_to_openai: add reasoing_content
• Converter.items_to_messages: just pass the reasoning item

[ericlu88]

@seratch Gentle ping on this PR. We've been using a fork of the repo to get this fix in. Would love to get back to main stream.

[seratch]

The essential change should be good to go but before reviews, could you fix the mypy errors?

uv run mypy .
src/agents/extensions/models/litellm_model.py:371: error: Unexpected keyword argument "reasoning_content" for "ChatCompletionMessage"  [call-arg]
Found 1 error in 1 file (checked 261 source files)
make: *** [Makefile:20: mypy] Error 1
Error: Process completed with exit code 2.

[zkllll2002]

The essential change should be good to go but before reviews, could you fix the mypy errors?

uv run mypy .
src/agents/extensions/models/litellm_model.py:371: error: Unexpected keyword argument "reasoning_content" for "ChatCompletionMessage"  [call-arg]
Found 1 error in 1 file (checked 261 source files)
make: *** [Makefile:20: mypy] Error 1
Error: Process completed with exit code 2.

@seratch thanks, done

[seratch]

Looks good to me; @rm-openai any thoughts?