chat : fix reasoning leak with force-opened bare <think> templates (#24674)

chat : fix reasoning leak with force-opened bare <think> templates (#24674)

* chat : fix reasoning leak with force-opened bare <think> templates

The reasoning start tag inferred from prior turns can carry trailing
whitespace (e.g. <think>\n) while a force-open template prefills a bare
<think>. Trim the tag used for the prefix split so the bare prefill is
matched instead of being swallowed into content.

* chat : fix Nemotron Nano v2 regression

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Co-authored-by: Alde Rojas <redacted>