Closed - Please ignore
Author: adi776borate
Pull request: yes

# What does this PR do?

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Fixes #43110 

This PR fixes an issue in the `MistralCommonBackend` tokenizer where special tokens (such as `[IMG]`, `
`, `[TOOL_CALLS]`) were incorrectly being split into multiple tokens instead of being mapped to their single, corresponding special token ID.

## Problem
The `MistralCommonBackend` wrapper was passing input text directly to the underlying `mistral_common` tokenizer's encode method. Unlike the `PythonBackend` or `Fast` tokenizers, it lacked a pre-tokenization step to identify and isolate special tokens. As a result, model-specific/extra special tokens were treated as regular string content and fragmented, leading to [this error](https://github.com/huggingface/transformers/issues/43110#issuecomment-3717684309).

## Solution
I have aligned the `MistralCommonBackend` implementation with the `PythonBackend`'s strategy for handling special tokens which is uses Trie-based pre-splitting. The input text is now split using the Trie and segments matching a special token are converted directly to their ID using a new `_special_token_to_id` helper.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?
@ArthurZucker @juliendenize 
Anyone in the community is free to review the PR once the tests have passed.

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Comments:
Closing this since upstream changes may already address the issue. I will reopen if needed on top of latest main.