Add unit tests

Add unit tests for critical areas, giving priority to following.

1. Preprocessing
2. Model training
3. Classifying

The issue mentions testing preprocessing, model training, and classifying. Look for existing test files or a tests/ directory to understand the project's testing pattern. Examine the codebase to locate the preprocessing, training, and classification modules. Write unit tests for these areas, ensuring they run with the project's test runner (likely pytest). Verify tests pass and cover the critical logic.

索引モデルが issue の本文から書いたものです。