Add a Model Config to Gym

The Gym pages are the reference; when they and this skill disagree, the pages win: Configuration (the services and driver fields), Multi-node vLLM and vLLM model server (the vllm_model fields, chat_template_kwargs, extra_body).

A model in a Gym evaluation has two parts:

• The deployment: a vllm service in the submit config, started with vllm serve, and named as the policy with driver.policy_model.
• The client: the model-type config Gym talks to it with (driver.policy_model_type, the --model-type of gym eval run). openai_model is the default; vllm_model adds reasoning extraction, chat_template_kwargs, extra_body and token information.

1. Read the model card first