Add comprehensive unit tests for services and main package

- Implement tests for the app service, including course processing from file and URI.
- Create mock implementations for CourseParser and Exporter to facilitate testing.
- Add tests for HTML cleaner service to validate HTML content cleaning functionality.
- Develop tests for the parser service, covering course fetching and loading from files.
- Introduce tests for utility functions in the main package, ensuring URI validation and string joining.
- Include benchmarks for performance evaluation of key functions.

go.sum

@@ -1,6 +1,6 @@
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