fix(provider): gate reasoning-to-content fallback behind spec flag

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

nanobot/providers/openai_compat_provider.py

@@ -759,8 +759,8 @@ class OpenAICompatProvider(LLMProvider):
             finish_reason = str(choice0.get("finish_reason") or "stop")
 
             raw_tool_calls: list[Any] = []
-            # StepFun Plan: fallback to reasoning field when content is empty
-            if not content and msg0.get("reasoning"):
+            # StepFun: fallback to reasoning field when content is empty
+            if not content and msg0.get("reasoning") and self._spec and self._spec.reasoning_as_content:
                 content = self._extract_text_content(msg0.get("reasoning"))
             reasoning_content = msg0.get("reasoning_content")
             if not reasoning_content and msg0.get("reasoning"):
@@ -820,7 +820,7 @@ class OpenAICompatProvider(LLMProvider):
                     finish_reason = ch.finish_reason
             if not content and m.content:
                 content = m.content
-            if not content and getattr(m, "reasoning", None):
+            if not content and getattr(m, "reasoning", None) and self._spec and self._spec.reasoning_as_content:
                 content = m.reasoning
 
         tool_calls = []

nanobot/providers/registry.py

@@ -71,6 +71,11 @@ class ProviderSpec:
     # "reasoning_split" — {"reasoning_split": true/false}  (MiniMax)
     thinking_style: str = ""
 
+    # When True, treat the "reasoning" response field as formal content
+    # when "content" is empty.  Only set this for providers (e.g. StepFun)
+    # whose API returns the actual answer in "reasoning" instead of "content".
+    reasoning_as_content: bool = False
+
     @property
     def label(self) -> str:
         return self.display_name or self.name.title()
@@ -325,6 +330,7 @@ PROVIDERS: tuple[ProviderSpec, ...] = (
         display_name="Step Fun",
         backend="openai_compat",
         default_api_base="https://api.stepfun.com/v1",
+        reasoning_as_content=True,
     ),
     # Xiaomi MIMO (小米): OpenAI-compatible API
     ProviderSpec(