(test) add unit tests for installer app hint logic

[avatGit]

What does this PR do?

As requested, this PR adds unit tests for the pure helper functions in src/sifty/core/junk.py that handle the installer app gating logic.

Changes

• Added test_installer_app_hint_strips_suffixes to verify version and architecture cleaning.
• Added test_is_installed_app_installer_matches_substrings to validate bidirectional substring matching using a frozenset.
• Added test_is_installed_app_installer_under_3_chars_guard to ensure the short-hint safety guard works perfectly.

Closes #29

Safety checklist

Sifty deletes files, so every PR keeps these promises:

• pytest is green, including tests/test_safety.py
• No new direct deletions (os.remove, shutil.rmtree, Path.unlink); everything goes through safety.trash()
• New destructive paths default to dry-run and ask before applying
• New core functions have a matching test

[github-actions]

Thanks for your first pull request to Sifty! A couple of things that help it land quickly: Sifty deletes files, so all deletion must go through safety.trash() and destructive paths default to dry-run. Please make sure pytest is green (especially tests/test_safety.py) and that the PR title starts with a type tag like (feat) or (fix). See CONTRIBUTING.md for the full checklist. A maintainer will review shortly.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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