vocab : honor BOS/EOS settings for PLaMo-2 and PLaMo-3 (#29734)

* vocab : honor BOS/EOS settings for PLaMo-2 and PLaMo-3

The original tokenizer configs for PLaMo-2 and PLaMo-3 have
`add_bos_token: true` and `add_eos_token: false` , but
_set_vocab_plamo() did not write the BOS/EOS metadata. The
PLAMO2 tokenizer path also ignored add_bos/add_eos during
tokenization.

Write the settings from tokenizer_config.json and honor them in
the PLAMO2 tokenization path. GGUFs without these keys keep the
previous behavior.

* Update conversion/base.py

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@huggingface.co>

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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@huggingface.co>