Add XCUI expansion state and coverage tests

engine/include/XCEngine/UI/Widgets/UIExpansionModel.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <cstddef>
+#include <string>
+#include <string_view>
+#include <unordered_set>
+
+namespace XCEngine {
+namespace UI {
+namespace Widgets {
+
+class UIExpansionModel {
+public:
+    bool HasExpandedItems() const;
+    std::size_t GetExpandedCount() const;
+    bool IsExpanded(std::string_view id) const;
+
+    bool SetExpanded(std::string itemId, bool expanded);
+    bool Expand(std::string itemId);
+    bool Collapse(std::string itemId);
+    bool ToggleExpanded(std::string itemId);
+    bool Clear();
+
+private:
+    std::unordered_set<std::string> m_expandedIds = {};
+};
+
+} // namespace Widgets
+} // namespace UI
+} // namespace XCEngine