This adds a `preserve_thinking` parameter to the chat template, controlling whether `<think>…</think>` reasoning blocks from previous assistant turns are kept when formatting a multi-turn conversation.
- **Default: `preserve_thinking = true`** — reasoning is kept in every assistant turn, exactly as before. The rendered output is unchanged for existing callers (verified byte-identical on multi-turn conversations, with and without a generation prompt).
- **`preserve_thinking = false`** — reasoning is removed from all assistant turns preceding the last user message; only the most recent turn keeps its `<think>` block. This shortens the prompt and reduces the KV-cache footprint in multi-turn settings. The mechanism reuses the template's existing `last_query_index` logic, matching the behavior introduced in Qwen3.6.
Usage:
```python
tokenizer.apply_chat_template(messages, preserve_thinking=False, ...)
```
or, when serving (e.g. vLLM), pass it through `chat_template_kwargs={"preserve_think…
