fix(sglang): force reasoning when the template prefills the think tag - #11790

Qwen3-style chat templates append the opening <think> tag to the *prompt*
when thinking is enabled. The model therefore never generates it and emits
only the reasoning text plus the closing </think>.

sglang's ReasoningParser keys off the opening tag:

    in_reasoning = self._in_reasoning or self.think_start_token in text
    if not in_reasoning:
        return StreamingParseResult(normal_text=text)

so with such a template the entire completion — reasoning and answer, the
raw </think> in between — is returned as content and reasoning_content
stays empty, no matter how reasoning_parser is configured.

sglang's own OpenAI server handles this via

    force_reasoning = (self.template_manager.force_reasoning
                       or self._get_reasoning_from_request(request))

This backend has no template manager, so derive the same signal from the
rendered prompt: if it ends with the detector's think_start_token, the tag
was prefilled and the parser is constructed with force_reasoning=True.

Structured decoding is the exception, and it matters: a grammar applies
from the first token, so the model cannot emit the closing tag even though
the template opened the block. The whole completion is schema output and
belongs in content — forcing there files it as reasoning and returns an
empty answer. Measured against a JSON-schema code audit: 10107 characters
of "reasoning", zero content. sglang's own server keeps the two apart for
the same reason; its grammar backend owns the reasoning prefix when a
reasoning parser is configured.

force_reasoning is only passed when it is meant to be True, so detector
defaults (DeepSeek-R1 already defaults to True) are untouched, and a
prompt without a prefilled tag behaves exactly as before — which matters,
because forcing unconditionally makes an answer generated with thinking
off disappear into reasoning_content.

The construction is factored into _new_reasoning_parser() so the streaming
and non-streaming paths, which previously built the parser separately,
cannot drift apart.

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