Introduces ICurrentUserSessionService and its implementation to provide access to the current user's session ID via the 'sid' claim. Registers the service in DI and adds comprehensive unit tests to validate correct and error scenarios.
Introduced ICurrentUserService and its implementation to centralize retrieval of the current user's ID from claims. Updated FriendsController to use this service instead of direct claim access. Registered the service and IHttpContextAccessor in DI. Added integration tests for FriendsController. Moved UsernameValidator to Infrastructure/Validators.