[Tests] Use QtTest for model contracts and signal spies (#7630)

Qt6::Test was linked into every test but never used. Add a small RAII
QAbstractItemModelTester helper and apply it to the deck list model, the card
database model and display proxy, and the sets model. Add QSignalSpy coverage
for the card-added and download-settings signals, and assert explicitly that a
card row is a leaf with no child columns (the tester does not check that).

The contract checker immediately found four model bugs, fixed here:
- DeckListModel allowed children below non-zero columns.
- CardDatabaseModel reported every card as a child of every card row.
- CardDatabaseModel reported six columns for a valid (leaf) card index.
- CardDatabaseDisplayModel claimed more rows were fetchable while rowCount
  already exposed them all, so fetchMore() inserted duplicates.

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

libcockatrice_models/libcockatrice/models/database/card_database_model.cpp

@@ -27,14 +27,18 @@ CardDatabaseModel::CardDatabaseModel(CardDatabase *_db, bool _showOnlyCardsFromE
 
 CardDatabaseModel::~CardDatabaseModel() = default;
 
-int CardDatabaseModel::rowCount(const QModelIndex & /*parent*/) const
+int CardDatabaseModel::rowCount(const QModelIndex &parent) const
 {
-    return cardList.size();
+    // A list model's card rows are leaves; returning the full card count for a
+    // valid parent makes every row claim all cards as children.
+    return parent.isValid() ? 0 : cardList.size();
 }
 
-int CardDatabaseModel::columnCount(const QModelIndex & /*parent*/) const
+int CardDatabaseModel::columnCount(const QModelIndex &parent) const
 {
-    return CARDDBMODEL_COLUMNS;
+    // Card rows are leaves, so a valid parent holds no columns. Reporting the
+    // row width for a card index would claim child cells that do not exist.
+    return parent.isValid() ? 0 : CARDDBMODEL_COLUMNS;
 }
 
 QVariant CardDatabaseModel::data(const QModelIndex &index, int role) const