Refactor Broadcast class and add unit tests

[scubaninja]

This pull request includes significant updates to the Broadcast model and adds unit tests to ensure the correctness of the model's behavior. The most important changes include adding documentation comments in Polish, modifying the Broadcast class structure, and introducing unit tests for the Broadcast class.

Updates to Broadcast model:

• Added detailed XML documentation comments in Polish to the properties and methods of the Broadcast class.
• Removed methods FromMarkdownEmail, FromMarkdown, and ContactCount from the Broadcast class.
• Made the constructor of the Broadcast class private to prevent direct instantiation from outside the class.

Addition of unit tests:

• Created a new BroadcastTest class in server/Models/tests.cs to test the default values, property setting, and handling of null values in the Broadcast class.