Increase code coverage from `55%` to `80%`

Current test coverage is at `55%`, which is below our target of `80%`. We need to add more unit tests, particularly for edge cases and critical paths, to improve reliability and maintainability. Focus on untested modules shown in the coverage report.

Start with the coverage report to identify the untested modules; no specific files or tests are named in the issue. Add unit tests for relevant edge cases and critical paths, then rerun coverage to verify that the project reaches the 80% target.

Written by the indexing model from the issue text.