[`fix`] Always early return for non-Mistral models in _patch_mistral_regex (#45444)
* Always early return for non-Mistral models in _patch_mistral_regex
Regardless of transformers version
* Revert to old approach, but use 5.0.0 as 5.0.0rc0 introduced the fix
So then we had 2 bugs:
1. 4.57.3 always warns
2. faulty mistral tokenizers saved with version 4.57.{3,4,5,6} will load incorrectly without a warning
* Exclusively check for mistral BEFORE v5
* Also link to this PR in test reference