auctionSellItem now resolves the item GUID internally via
backpackSlotGuids_ with resolveOnlineItemGuid fallback, matching the
pattern used by vendor sell and item use. Previously the UI passed
the GUID directly from getBackpackItemGuid() with no fallback, so
items with unset slot GUIDs silently failed to list.
Also gates CMSG_AUCTION_SELL_ITEM format by expansion: Classic/TBC
omits the itemCount and stackCount fields that WotLK requires.