Handling Responses and Errors of Authentication

[Hazem-0]

This PR improves the robustness of the authentication system. It prevents users from logging in from multiple devices simultaneously, handles database connection failures gracefully during auth, and fixes the response flow for user registration.

Key Changes:

Concurrent Login Prevention: Added isPlayerOnline check in GameServerManager and AuthenticationService. If a user is already online, the server now returns a USER_IS_ONLINE error action instead of allowing a second session.

Database Error Handling: Updated PlayerDAO and AuthenticationService to properly catch SQLException and handle null database connections. The server now returns an INTERNAL_SERVER_ERROR message to the client if the DB is down, rather than crashing or hanging.

Registration Flow: Fixed MessageRouter to correctly send the registration response message back to the client.

Refactoring:

Improved thread safety and logic in getAvailablePlayersMessage (GameServerManager).

Added logic to ensure the requesting player is not included in their own "Available Players" list.