[Mistral] Add tekken tokenizer detection, conversion, and save utilities

[juliendenize]

What does this PR do?

Builds on #46603.

• Detect native Mistral checkpoints with tri-state control (resolve_mistral_format).
• Build an HF fast tokenizer from a tekken.json (convert_tekken_tokenizer).
• Reconstruct a tekken.json from an HF tokenizer, losslessly (save_as_tekken).
• Build a PixtralProcessor from native tekken.json + params.json (convert_tekken_image_processor).

Tested with RUN_SLOW=1 pytest tests/integrations/mistral/test_tokenizer.py → 38 passed, including a slow real-file tekken → HF → tekken round-trip and mistral-common cross-backend equivalence.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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tokenizers: @ArthurZucker and @itazap

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https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=46604&sha=9fe4f2