Refactor Tunix adapter to use a dedicated weight mapping utility.

[abhinavclemson]

PiperOrigin-RevId: 825825400

Description

Refactor Tunix adapter to use a dedicated weight mapping utility.

This change introduces VllmWeightMapping in tunix/utils.py to encapsulate the logic for mapping MaxText weights to vLLM-compatible formats. The TunixMaxTextAdapter now uses an instance of VllmWeightMapping, which includes support for sharding knowledge and model-specific hook functions. New hook functions for Llama3.1 and Qwen3 are added to param_mapping.py

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