🧪 test: add boundary tests for slugify function

[masuda-so]

Added unit tests for the slugify pure function in scripts/distill.py to cover standard and boundary conditions, improving coverage and reliability.

What: Added pytest-based unit tests for distill.slugify
Coverage: Normal inputs, empty strings, spaces/special characters, long string truncation (80 chars limits), hyphen trimming
Result: Improved codebase test coverage providing an effective safety net for refactoring.

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PR created automatically by Jules for task 782021141816324965 started by @masuda-so

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