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

• chat : fix reasoning leak with force-opened bare templates

The reasoning start tag inferred from prior turns can carry trailing whitespace (e.g. \n) while a force-open template prefills a bare . 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

Co-authored-by: Alde Rojas [email protected]