StepFun Plan API returns response content in the 'reasoning' field when
the model is in thinking mode and 'content' is empty. OpenAICompatProvider
previously only checked 'content' and 'reasoning_content', missing this field.
This patch adds a fallback: if content is empty and 'reasoning' is present,
extract text from reasoning to populate content, ensuring StepFun models
(step-3.5-flash, step-3.5-flash-2603) work correctly with tool calls.
Co-authored-by: moranfong <moranfong@gmail.com>