Qwen-style chat templates pre-fill the opening <think> in the prompt, so the
model emits only the reasoning body + a bare closing </think> — and they think
by DEFAULT regardless of the API enable_thinking flag. The old paired-tag
reasoning extractor missed the bare close, leaking the whole thought (and the
</think>) into content and conversation history.

- extract_reasoning_content: handle a bare </think|/thinking|/thought> with no
  opening tag (treat the prefix as reasoning).
- streaming: a chunk-safe reasoning gate routes the thought into
  delta.reasoning / reasoning_content until </think>, then flips to content;
  tool extraction runs on the post-</think> answer only.
- non-streaming: extract reasoning, set message.reasoning(+_content), clean
  content; tools parsed from the answer.
- activate whenever the model auto-thinks (qwen3/qwq/deepseek-r1/… name) OR
  reasoning is explicitly enabled — not just on the API flag.
- configurable suppression: per-model `suppress_reasoning`, or per-request via
  the standard reasoning:{exclude:true} / reasoning_effort:"none" /
  suppress_reasoning fields. Emits both `reasoning` and DeepSeek-style
  `reasoning_content` for client compatibility.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>