[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>

tests/support/model_contract_checker.h

@@ -0,0 +1,35 @@
+#ifndef COCKATRICE_TEST_MODEL_CONTRACT_CHECKER_H
+#define COCKATRICE_TEST_MODEL_CONTRACT_CHECKER_H
+
+#include <QAbstractItemModel>
+#include <QAbstractItemModelTester>
+#include <memory>
+
+namespace test_support
+{
+/**
+ * @brief Keeps a QAbstractItemModelTester alive for the lifetime of a scope.
+ *
+ * The tester only observes the model while it exists, so construct this before
+ * the mutations a test wants to validate and keep it in scope until the end.
+ * Fatal reporting mode aborts on the first contract violation; a warning would
+ * let the test pass while printing a line nobody reads in CI.
+ */
+class ModelContractChecker
+{
+public:
+    explicit ModelContractChecker(QAbstractItemModel *model)
+        : tester(
+              std::make_unique<QAbstractItemModelTester>(model, QAbstractItemModelTester::FailureReportingMode::Fatal))
+    {
+    }
+
+    ModelContractChecker(const ModelContractChecker &) = delete;
+    ModelContractChecker &operator=(const ModelContractChecker &) = delete;
+
+private:
+    std::unique_ptr<QAbstractItemModelTester> tester;
+};
+} // namespace test_support
+
+#endif // COCKATRICE_TEST_MODEL_CONTRACT_CHECKER_H