[Mistral] Move MistralConverter into integrations/mistral/ package
Author: juliendenize
Pull request: yes

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# 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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## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

tokenizers: @ArthurZucker and @itazap

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_46603). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Thank you for your contribution 🤗!

## CI Security Gate — automatic approval blocked

This PR was **not** automatically approved for CI because the security gate failed.

**Possible reasons:**
- The PR touches 50 or more files — only PRs with fewer than 50 changed files are automatically approved
- A changed file is outside the allowed directories (`src/`, `tests/`, `docs/`, `utils/`), has a disallowed extension (only `.py`, `.txt`, `.md` permitted outside `tests/` and `docs/`), or is not `.md`/`.yml` inside `docs/`
- A new high-severity security issue was detected in the changed Python files (Bandit check)

See the [workflow run](https://github.com/huggingface/transformers/actions/runs/27988544646) for the exact violations.

A maintainer can review and manually approve CI if a finding is a false positive.

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: afmoe, aimv2, albert, align, altclip, apertus, arcee, aria, audio_spectrogram_transformer, audioflamingo3, auto, autoformer, aya_vision, bamba, bark, bart

@ArthurZucker hopefully i answered to your comments 😄 

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### CI recap

**Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=46603)
**Latest run:** [28948624437:2](https://github.com/huggingface/transformers/actions/runs/28948624437)
**Result:** `success` | **Jobs:** 15 | **Tests:** 172,394 | **Failures:** 0 | **Duration:** 7h 27m

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