Disable thinking mode in vLLM instruct chat

[dmndxld]

Problem

Qwen3 enables thinking mode by default in its chat template, generating <think> blocks before the actual response.
This interferes with uncertainty estimation, as the generated text contains reasoning traces instead of direct
answers.

When using WhiteboxModelvLLM with instruct=True, the model.chat() call invokes apply_chat_template internally,
which enables thinking mode by default for Qwen3.

Fix

Pass chat_template_kwargs={"enable_thinking": False} to model.chat(), which propagates to apply_chat_template
and disables thinking mode:

output = self.model.chat(
    *args, chats, sampling_params,
    chat_template_kwargs={"enable_thinking": False},
)

This uses vLLM's native chat_template_kwargs parameter rather than manually calling apply_chat_template +
model.generate(), keeping the implementation consistent with vLLM's API.

Affected models

Models that enable thinking mode by default in their chat template, such as Qwen3.