Write unit test cases for all functions

Add comprehensive unit tests for all functions to ensure reliability and maintainability.
The tests should mock the `node-appwrite` SDK to avoid real API calls and verify that each function:

* Calls the correct Appwrite methods with expected parameters.
* Returns the correct values on success.
* Handles and propagates errors properly.

Use **Jest** for testing, and include both positive and negative scenarios for full coverage.