fix(openai-adapter): map max_tokens to max_completion_tokens for DeepSeek reasoner (#11889)

DeepSeek Reasoner models require max_completion_tokens instead of max_tokens,
similar to OpenAI's o-series models. This maps the parameter automatically
when the API base is api.deepseek.com or the model name includes
"deepseek-reasoner".

Co-authored-by: Burak Bebek <BurakBebek1@users.noreply.github.com>