[SENSOR] Write unit tests for all sensor classes

[ezehf2]

Create a tests/ folder and write a pytest test file for each sensor class. Each test file should check that the generate_reading() method returns valid JSON, that the value falls within the expected range, that the sensor type and room fields are present and correct, and that consecutive readings show drift rather than random jumps. Run all tests with pytest and confirm they pass before closing this issue. This is part of the QA Lead responsibility and will be checked during assessment.