Add tests

[polyedr]

This PR introduces initial pytest-based tests for several helper modules:

• bucket_tools: verify nearest bucket selection and bucket table consistency
• bucket_table: sanity check that bucket options are non-empty and well-formed
• clip_vision: smoke test for hf_clip_vision_encode with dummy extractor/encoder
• hf_login: tests retry logic and absence of autologin when HF_TOKEN is unset
• memory: test DynamicSwapInstaller install/uninstall behavior with stubbed torch

All tests are lightweight and self-contained. They use stubs to avoid requiring
heavy dependencies (torch, diffusers, etc.) and can be run with:

pip install pytest
python -m pytest -q

This should make it easier to validate basic functionality of these utilities
without GPU or large model downloads.