fix(qwen): always send explicit enable_thinking in chat_template_kwargs

[LESdylan]

Problem

skillopt/model/qwen_backend.py only attached chat_template_kwargs when enable_thinking was true:

if config.enable_thinking:
    payload["chat_template_kwargs"] = {"enable_thinking": True}

With enable_thinking=false the key was omitted entirely, so whether Qwen thinking mode ran was decided by the vLLM server / chat-template default — which varies across deployments and makes results hard to reproduce. This is exactly point 3 of #90, where @Huangzisu confirmed the reported results used client-side enable_thinking=false and that the code would be adjusted.

Change

Always send an explicit boolean:

payload["chat_template_kwargs"] = {"enable_thinking": bool(config.enable_thinking)}

This matches the MiniMax backend, which already sends the flag unconditionally (minimax_backend.py).

Tests

Updated the two tests that pinned the omit-when-disabled behavior to assert an explicit {"enable_thinking": False} instead. Full suite: 165 passed, 5 skipped.

Ref #90

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