fix(mail): route GO mailbox open through InventoryHandler

The decomposition PRs moved mail state to InventoryHandler but the GO
interaction code still set stale GameHandler fields. Add openMailbox()
on InventoryHandler and forward from GameHandler so the correct
mailboxGuid_/mailboxOpen_ are set and refreshMailList() works.

include/game/game_handler.hpp

@@ -1981,6 +1981,7 @@ public:
     void openMailCompose();
     void closeMailCompose();
     bool hasNewMail() const;
+    void openMailbox(uint64_t guid);
     void closeMailbox();
     void sendMail(const std::string& recipient, const std::string& subject,
                   const std::string& body, uint64_t money, uint64_t cod = 0);