[Fix] Add missing init_kwargs to MistralCommonBackend for v5 processor compatibility
Author: adi776borate
Pull request: yes

# What does this PR do?

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This PR fixes a bug in `MistralCommonBackend` (introduced in v5) where the `init_kwargs` attribute was missing. This caused crashes when using the tokenizer with processors like `PixtralProcessor`, which rely on this attribute to merge configurations.

Specifically, it adds the manual initialization of `self.init_kwargs` in `MistralCommonBackend.__init__`.

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Fixes #43110 
Please refer to [Related vLLM issue](https://github.com/vllm-project/vllm/issues/31792#issuecomment-3717160427) for the error traceback.

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Comments:
Can this be merged?