Add support for returning reasoning_text

[hwichan0720]

This PR adds support for retrieving reasoning_text from language model.
To achieve this, we introduce the following changes:

• Add a reasoning_text field to the LMOutput data class
• During generation, extract res.choices[0].message.reasoning_content and store it in this field.

Note: Currently, this modification is applied only to vllm_serve_lm (openai_api), not to vllm_model or hf_lm. As a result, reasoning_text is available only when using vllm_serve_lm.

[junya-takayama]

LGTM!