The non-streaming parse path unconditionally promoted the `reasoning`
response field to `content` when content was empty. This was intended
for StepFun (whose API returns the actual answer in `reasoning`), but
it applied to every OpenAI-compatible provider — causing internal
thinking chains from models like Xiaomi MIMO to be leaked as formal
replies.
Add `reasoning_as_content: bool` to ProviderSpec (default False) and
set it only for StepFun. The fallback now requires this flag rather
than running globally.
Fixes#3443
- Fix assertion in streaming dict fallback test (trailing space in data
not reflected in expected value).
- Add two regression tests proving that models with reasoning_content
(e.g. DeepSeek-R1) and standard models (no reasoning fields) are
completely unaffected by the reasoning fallback.
Made-with: Cursor
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
