AutoTokenizer: clear ImportError when loading Voxtral without mistral-common + unit test

[Elon7069]

Summary: Fix confusing TypeError from SentencePiece when loading Voxtral tokenizer without mistral-common by raising a clear ImportError instead. Fixes #41553.
Rationale: Without mistral-common, loading via AutoTokenizer ends in a low-level TypeError inside sentencepiece. Users should see a direct, actionable message.
What’s changed
AutoTokenizer guard:
In AutoTokenizer.from_pretrained, after resolving config and before class selection, raise a clear error if config.model_type == "voxtral" and mistral-common is missing.
Message:
"The Voxtral tokenizer requires the 'mistral-common' package. Please install it using pip install mistral-common."
Tests:
Added tests/models/voxtral/test_tokenization_voxtral.py
Mocks is_mistral_common_available to False and get_tokenizer_config to avoid network, then asserts ImportError mentioning "mistral-common".
Why this approach
Keeps mapping logic intact; avoids unexpected fallbacks.
Ensures the user sees a clear, actionable message as early as possible in the loading path.
Testing
Targeted test: pytest tests/models/voxtral/test_tokenization_voxtral.py -q → passes.
No network calls thanks to test monkeypatching.
Backward compatibility
No behavior change when mistral-common is installed.
Only affects Voxtral when the dependency is missing.

[github-actions]

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

run-slow: auto, voxtral

[Elon7069]

Maintainer please check my PR lemme know what i have to improve more

[Rocketknight1]

I think this is a duplicate of #41592!

[Elon7069]

I think both are different ki lemme check> I think this is a duplicate of #41592!