fix(vllm): tell the reasoning parser whether thinking was enabled

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vLLM's engine-based reasoning parsers derive their initial state from the
chat template kwargs. Qwen3Parser:

    chat_kwargs = kwargs.get("chat_template_kwargs", {}) or {}
    self.thinking_enabled = chat_kwargs.get("enable_thinking", True)

Constructed as ReasoningParser(tokenizer) the flag defaults to True, so the
parser starts in the REASONING state. A completion produced with thinking
disabled contains no tags at all, and every reasoning parser shape then
reports the whole answer as reasoning:

  - engine-based parsers classify it by initial state;
  - BaseThinkingReasoningParser hits its documented "may not generate start
    token" fallback and returns (model_output, None).

Either way `content = c if c is not None else generated_text` turns that
into a duplicate: a Qwen3 model answering "391" with thinking off comes back
as reasoning_content="391" AND content="391".

Measured against Qwen3.5-MoE on vLLM 0.28, non-streaming:

    before   thinking on   reasoning=202  content="391"
             thinking off  reasoning="391" content="391"   <- duplicated
    after    thinking on   reasoning=192  content="391"
             thinking off  reasoning=""    content="391"

Forward the kwargs the prompt was rendered with, which is what vLLM's own
OpenAI server does; parsers that do not accept the argument keep the plain
constructor.

_split_reasoning() covers the older parser shape, which has no initial state
to set. It only reclassifies when the parser exposes a start/end token pair
and neither the completion nor the prompt ever opened a reasoning block.
Truncated reasoning (block open, end token never arrived) stays reasoning,
and parsers without that token pair are left untouched.

Signed-off-by: pos-ei-don <1822533+pos-ei-don@users.noreply.github.com>

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