Implement comprehensive unit tests for core services and models

[InfoTCube]

Description:
Establish a robust unit testing suite for the Liborrow backend to ensure individual components work correctly in isolation, focusing on book management, friend interactions, and loan tracking.

Tasks:

• Set up xUnit testing project
• Add Moq/FakeItEasy for mocking dependencies
• Test BookService (ISBN lookup, catalog management)
• Test FriendService (requests, accept/decline logic)
• Test LoanService (borrow requests, returns, tracking)
• Test JWT authentication/authorization logic
• Mock Biblioteka Narodowa API calls
• Achieve minimum 70% code coverage
• Add test CI step in GitHub Actions

Acceptance Criteria:

• Core services have >70% test coverage
• Tests run automatically on PRs
• Mocked external API calls don't hit real endpoints