Add tests for DemandApi endpoints

[14Richa]

Description

This PR adds unit tests for the DemandApi class and covers various endpoints related to demand data, including actual total load, daily outturn, and streaming data. The new tests cover scenarios including successful API calls, empty responses, error handling, and parameter validation.

Changes

Added following test methods in the TestDemandApi class:

1. test_demand_actual_total_get
2. test_demand_outturn_daily_get
3. test_demand_outturn_daily_stream_get
4. test_demand_outturn_daily_stream_get_empty
5. test_demand_outturn_daily_stream_get_error
6. test_demand_outturn_get
7. test_demand_outturn_get_empty
8. test_demand_outturn_get_error
9. test_demand_outturn_get_with_parameters

Testing

To run the new tests:

1. Navigate to the project directory
2. Run python -m unittest test_demand_api.py

Checklist:

• [X] My code follows OCF's coding style guidelines
• [X] I have performed a self-review of my own code
• [X] I have checked my code and corrected any misspellings

[peterdudfield]

Also, could you run the tests with pytest please

[peterdudfield]

Ill close this, as we decided to do it a different way