model-conversion : add --add-bos to run org model script (#29558)

This commit adds an optional --add-bos token command line option to the
run-org-model.py script.

The motivation for this is that there are models, for example Gemma4,
that explicitely set the add_bos value to true in llama-vocab.cpp even
if the original model does not set this value to True.

It would be nice to be able to force the models to agree on the bos
token so that logit verification can proceed.

Refs: https://github.com/ggml-org/llama.cpp/pull/21500