Add unit and integration tests to have test coverage

[bakonyiferenc]

Currently, this repository does not have any real tests. I propose to add tests to improve the code quality, prevent regression and ensure reliability of the project. It would make future contributions (PRs, refactoring, bug fixes) safer and easier to review.

Scope:

• Write tests for existing functionality that currently lacks coverage.
• Aim for a reasonable baseline of coverage (core functionalities, error handling, etc.).

Please let me know if you have preferences about tools (e.g., pytest) or specific areas/features to prioritize in testing. I am happy to discuss further before starting.

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If approved, I will begin by setting up a basic test structure and open separate PR(s) for the tests.

[iMicknl]

Great suggestion, @bakonyiferenc! This would definitely be a great addition. Pytest would be my preference. Happy to review any contributions.

The only difficulty might be to mock device behavior, as I have seen different behavior in Sagemcom devices... But here we would need to make assumptions.

[bakonyiferenc]

@iMicknl please review this approach.