Make tokenization_mistral_common importable without mistral_common installed

[juliendenize]

What does this PR do?

tokenization_mistral_common.py referenced mistral_common-only symbols at import time, so the module raised NameError when mistral_common was not installed. Those symbols are only imported inside the if is_mistral_common_available(): block, so they are undefined at module scope otherwise. This is not reachable through normal AutoTokenizer usage today since the module is only imported behind an is_mistral_common_available() guard, but this makes the module safe to import unconditionally.

The import-time references were the module-level _MAP_SPECIAL_TOKENS dict, the mode default arguments (ValidationMode.test, now the equivalent string "test"), and a few signature annotations referencing ValidationMode / ReasoningEffort (now quoted since the module has no from __future__ import annotations). Added a test that executes the module body with mistral_common both available and unavailable to ensure the import never raises.

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Who can review?

@ArthurZucker @itazap

[github-actions]

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CI recap

Dashboard: View test results in Grafana
Latest run: 29640458203:2
Result: success | Jobs: 7 | Tests: 498 | Failures: 0 | Duration: 3m 50s

[Rocketknight1]

looks ready for review so gentle ping @ArthurZucker @itazap

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[itazap]

Thanks for catching!