test(gguf): Add bfloat16 support to GGUF loading tests

[Rudra-Tiwari-codes]

Proposed changes

This PR extends the GGUF loading tests to support bfloat16 data types. Previously, bfloat16 was missing from the supported_dtypes list in tests/test_load.py, leaving a gap in test coverage for models utilizing this precision format. This change ensures that GGUF save and load operations are correctly validated for bfloat16.

Checklist

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• [ x] I have read the CONTRIBUTING document
• [ x] I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• [ x] I have added tests that prove my fix is effective or that my feature works
• [ x] I have updated the necessary documentation (if needed)