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
9b2f452b6e