tests: enable bfloat16 support for npy and safetensors

[shivansh023023]

Proposed changes

This PR addresses the missing bfloat16 testing parity for NPY, NPZ, and Safetensors formats by resolving an existing TODO in python/tests/test_load.py.

Changes included:

• Implemented a _to_np_dtype helper to correctly map bfloat16 to np.float32 during numpy conversions, allowing test arrays to be properly initialized and validated.
• Added "bfloat16" to the list of tested dtypes in test_save_and_load, test_save_and_load_fs, and test_savez_and_loadz.
• Note: bfloat16 remains intentionally excluded from test_save_and_load_gguf, as the C++ backend does not yet support it for the GGUF format.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed) (Not applicable)

[zcbenz]

Tests not passing, code not doing exactly what it claims, not understanding why we didn't have it in the first place.