[Mistral] Move MistralConverter into integrations/mistral/ package

[juliendenize]

What does this PR do?

• Move the tekken→HF tokenizer converter into a dedicated, lazily-loaded Mistral integration package.
• Fix the vocab index offset so special tokens occupy the first ids.
• Add from_tekken_file(), which pre-computes the vocab and merges in a single pass.
• Assign special-token ids from their explicit rank, independent of list ordering.
• Register special tokens as special so they are stripped when decoding with skip_special_tokens=True.
• Preserve the original tekken metadata so the tokenizer can be re-saved losslessly.

Tested with RUN_SLOW=1 pytest tests/integrations/mistral/test_tokenizer.py → 22 passed (incl. slow tests downloading tekken.json from mistralai/Ministral-3-3B-Instruct-2512 and mistral-common cross-backend equivalence).

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tokenizers: @ArthurZucker and @itazap

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