fix(Core/Battlegrounds): fill existing BGs instead of starting duplicates (#26585)

src/server/game/Battlegrounds/BattlegroundMgr.cpp

@@ -948,7 +948,7 @@ BGFreeSlotQueueContainer& BattlegroundMgr::GetBGFreeSlotQueueStore(BattlegroundT
 
 void BattlegroundMgr::AddToBGFreeSlotQueue(BattlegroundTypeId bgTypeId, Battleground* bg)
 {
-    bgDataStore[bgTypeId].BGFreeSlotQueue.push_front(bg);
+    bgDataStore[bgTypeId].BGFreeSlotQueue.push_back(bg);
 }
 
 void BattlegroundMgr::RemoveFromBGFreeSlotQueue(BattlegroundTypeId bgTypeId, uint32 instanceId)

src/server/game/Battlegrounds/BattlegroundQueue.cpp

@@ -805,8 +805,7 @@ void BattlegroundQueue::BattlegroundQueueUpdate(uint32 diff, BattlegroundTypeId
         }
     };
 
-    // battleground with free slot for player should be always in the beggining of the queue
+    // fill existing joinable battlegrounds, oldest first, before a new instance may form below
-    // maybe it would be better to create bgfreeslotqueue for each bracket_id
     BGFreeSlotQueueContainer& bgQueues = sBattlegroundMgr->GetBGFreeSlotQueueStore(bgTypeId);
     for (BGFreeSlotQueueContainer::iterator itr = bgQueues.begin(); itr != bgQueues.end();)
     {

src/server/game/Handlers/BattleGroundHandler.cpp

@@ -554,6 +554,8 @@ void WorldSession::HandleBattleFieldPortOpcode(WorldPacket& recvData)
             // leaks forever, permanently skewing team selection and blocking the
             // empty instance's cleanup.
             bg->DecreaseInvitedCount(teamId);
+            if (bg->HasFreeSlots())
+                bg->AddToBGFreeSlotQueue();
             _player->RemoveBattlegroundQueueId(bgQueueTypeId);
             _player->SetBattlegroundId(0, BATTLEGROUND_TYPE_NONE, PLAYER_MAX_BATTLEGROUND_QUEUES, false, false, TEAM_NEUTRAL);