fix: guard huffman import in test_preprocessing

[bugparty]

Summary

• Replaced top-level import huffman with huffman = pytest.importorskip("huffman") inside test_huffman, so the test file can be collected even when the huffman package is not installed
• Added import pytest (needed for importorskip)

Test plan

• Verified file syntax is valid
• Verified no top-level import huffman remains
• Verified pytest.importorskip("huffman") is present in test_huffman
• Full test suite run blocked by pre-existing Python 3.13 incompatibility (collections.Hashable removal, unrelated to this change)

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