Fix _set_model_specific_special_tokens to accept list-format extra_special_tokens

[bensons]

What does this PR do?

Some model repos provide extra_special_tokens as a list in their tokenizer_config.json, which caused an AttributeError: 'list' object has no attribute 'keys'. This converts list inputs to a dict mapping each token to itself before processing.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

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Who can review?

@Cyrilvallez
@itazap
@ArthurZucker

[Cyrilvallez]

Please provide the excat model where this happens before we review. Also, we do not want new test files

[bensons]

Example that caused me to find this behavior:
https://huggingface.co/jedisct1/Qwen3-VL-Embedding-8B-mlx/blob/main/tokenizer_config.json

[itazap]

I'm able to load AutoTokenizer.from_pretrained("jedisct1/Qwen3-VL-Embedding-8B-mlx") without any errors. Can you please pull on main and share the full reproducer and error you have?