Add unit tests for WebApi

[baratgabor]

The backend entry point, the WebApi project, is currently not tested.

Since this is an extremely shallow layer that is almost solely responsible for mapping an URI to an execution path, simple happy path unit tests will suffice.

Tasks:

• Add a new unit testing project for WebApi.
• Create happy path unit tests for the controller actions.
• Include the new test project in the CI-CD pipelines and the coverage reporting.
• Make sure that the WebApi assembly coverage is 90%+.
• If needed, cover additional components too besides the controllers.