feat: added test files for helper function

[souvikchand]

🧪 added test cases for custom_components/spoolman/helpers/add_trailing.sh

• created tests/ folder
• wrote 4 test cases for add_trailing_slash() function
• mocked the import of homeassistant
• so I added tests/utils/load_module.py

🔗 Related Issue

🎨 Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• 🧹 Code refactoring (no functional changes)
• ⚡ Performance improvement
• ✅ Test update
• 🔧 Configuration change
• 🎨 UI/UX improvement

🧪 Testing

• I have tested this code locally
• I have added/updated unit tests that prove my fix/feature works
• All existing tests pass locally
• I have tested this in Home Assistant

Test Environment

• Home Assistant version:
• Python version: 3.10.0
• Integration version:
• Spoolman version: 1.2.1

Test Scenarios

1. if input doesn't end with '/'
2. if input ends with '/'
3. if input is empty
4. if input is '/'

✅ Checklist

• 📖 My code follows the project's coding style
• 🔍 I have performed a self-review of my code
• 💬 I have commented my code, particularly in hard-to-understand areas
• 📚 I have updated the documentation (README, CONTRIBUTING, etc.) if needed
• ⚠️ My changes generate no new warnings or errors
• 🧪 I have added tests that prove my fix is effective or that my feature works
• ✅ New and existing unit tests pass locally with my changes
• 🌿 I am merging into the dev branch (not main)
• 📝 My commits follow the Conventional Commits specification
• 🔄 I have updated the integration version in manifest.json if needed
• 🌍 I have updated translations if I changed user-facing strings

📋 Commit Message Format

• feat added test files for helper function

📝 Additional Notes

I Mocked importing home assistant because the function was a simple oneand not dependent on any module. I hope that's acceptable

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For Maintainers

Review Checklist

• Code quality is acceptable
• Tests are adequate
• Documentation is updated
• Breaking changes are documented
• Ready to merge to dev

Post-Merge Tasks

• Update CHANGELOG if needed
• Tag for release (if applicable)
• Update documentation site (if applicable)

[souvikchand]

@Disane87 please check this PR regarding #269