🚀 Add more unit tests for the project

Is your feature request related to a problem? Please describe.
Currently there are still many parts of the project that aren't covered by unit tests yet.

Describe the solution you'd like
I'd like to see more unit tests added. I acknowledge this is a very big task, I'd split this task into smaller chunks that can be done at one time thus easier to review, e.g. add a unit test for a case for an existing function.

For the next step, we can add a unit test coverage checker in this project's github action workflow, so that the actions can check what part of the project that's still missing unit tests and avoid any coverage percentage decrease (this can be caused by either deleting existing unit tests, or adding a feature without unit test)

Start by surveying the existing Python unit tests and project areas that lack coverage; the issue does not name specific files or test entry points. Then inspect the GitHub Actions workflow for where coverage checking could fit. Done means a smaller, explicitly scoped test improvement or a defined coverage-checking change, rather than the entire broad request.