refactor(network): replace HandCard with std::multiset<Card> in InitGamePayload

- Updated `InitGamePayload` to use `std::multiset<Card>` for `handCard`.
- Removed `serializeHandCard` method and replaced its usage with `serializeCards`.
- Adjusted `deserializeHandCard` to return `std::multiset<Card>` and simplified insertion logic.
- Cleaned up unused declarations in `MessageSerializer`.

src/network/Message.h

@@ -35,7 +35,7 @@ namespace UNO::NETWORK {
 
     struct InitGamePayload {
         GAME::DiscardPile discardPile;
-        GAME::HandCard handCard;
+        std::multiset<GAME::Card> handCard;
         size_t currentPlayerIndex;
     };