Add comprehensive tests for the StepFun Plan API compatibility fix:
- _parse dict branch: content and reasoning_content fallback to reasoning
- _parse SDK object branch: same fallback for pydantic response objects
- _parse_chunks dict branch: reasoning field handled in streaming mode
- _parse_chunks SDK branch: reasoning fallback for SDK delta objects
- Precedence tests: reasoning_content field takes priority over reasoning
Refs: fix(provider): support StepFun Plan API reasoning field fallback
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>
moranfongandXubin Ren