Add contest registration and problem management features

[SeifMohmmed]

Changes:

• Introduced endpoints for registering users in contests and adding problems to contests in ContestController, with proper
• authorization and error handling.
• Added RegisterInContestCommand and AddContestProblemCommand with their respective handlers to
• encapsulate the logic.
• Added ICurrentUserService and IUserContestRepository interfaces with their implementations to manage user-contest relationships and current user retrieval.
• Refactored CreateContestCommandHandler and GetContestQueryHandler to use the new ICurrentUserService abstraction, replacing IHttpContextAccessor.
• Enhanced IContestRepository with methods for managing contest problems, including adding problems, checking duplicates, and retrieving problem counts.
• Updated Result and Result<T> to support nullable Error and Message properties for better flexibility.
• Improved caching logic in GetContestQueryHandler and fixed a bug in ProblemMappings.
• Registered new services in DependencyInjection and added validation for RegisterInContestCommand.

These changes improve functionality, usability, and maintainability.