Preserve assistant reasoning aliases

[rifoagenadi]

Summary

• preserve assistant message reasoning aliases through ChatCompletionRequest serde
• covers reasoning_content, think, think_fast, and think_faster, including empty-string values
• verify reasoning and reasoning_content can both survive serialization

Test

• env -u VIRTUAL_ENV -u VIRTUAL_ENV_PROMPT PYO3_PYTHON=/mnt/weka/home/rifo.genadi/miniconda3/bin/python OPENSSL_DIR=/mnt/weka/home/rifo.genadi/miniconda3 cargo test --test test_extra_args_chat