Add unit tests for param_mapping.py

[bvandermoon]

Description

This PR adds unit tests for param_mapping.py to increase code coverage. This change introduces tests for mapping and hook functions across various models (Gemma, Qwen, DeepSeek, etc.) and configurations (scanned vs unscanned layers).

Tests

Tested by running pytest on a TPU VM with coverage enabled:

pytest --cov=src/maxtext/checkpoint_conversion/utils tests/unit/param_mapping_test.py

Checklist

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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