fix: drop redundant reasoning_effort for Kimi thinking models

Moonshot's API rejects requests that carry both 'reasoning_effort'
(top-level kwarg) and 'thinking' (extra_body) at the same time.
After the unified thinking-style injection loop injects the native
'thinking' param for kimi models, pop 'reasoning_effort' from kwargs
since it is redundant and causes a 400 error.

Uses _model_slug() + _KIMI_THINKING_MODELS lookup to stay consistent
with the refactored code (the old _is_kimi_thinking_model helper was
removed in 4f895e63).

Existing kimi tests updated to assert 'reasoning_effort' is absent.
Xiaomi MiMo models are unaffected — their API accepts both params.

Closes #3939

nanobot/providers/openai_compat_provider.py

@@ -633,6 +633,14 @@ class OpenAICompatProvider(LLMProvider):
                 if extra:
                     kwargs.setdefault("extra_body", {}).update(extra)
 
+            # Moonshot rejects requests that carry both 'reasoning_effort'
+            # and the native 'thinking' param.  We already expressed the
+            # user's intent via the provider-native shape, so drop the
+            # redundant wire-level kwarg.  Only kimi models need this —
+            # Xiaomi's API accepts both params.
+            if _model_slug(model_name) in _KIMI_THINKING_MODELS:
+                kwargs.pop("reasoning_effort", None)
+
         if tools:
             kwargs["tools"] = tools
             kwargs["tool_choice"] = tool_choice or "auto"