feat: Add LCOV coverage support for Jenkins CI

- Add ENABLE_COVERAGE CMake option with gcov flags
- Create collect-coverage.sh script for LCOV workflow
- Add Qt6 test files with QTest framework
- Update Jenkinsfile with Coverage stage (main/master/coverage.*)
- Update CI Docker image with lcov and genhtml packages
- Add .gitignore entries for coverage artifacts
- Add COVERAGE.md documentation

Coverage workflow:
1. Configure with -DENABLE_COVERAGE=ON
2. Build and run tests to generate .gcda/.gcno files
3. Collect coverage with lcov
4. Filter out Qt/system libraries
5. Generate HTML report and lcov.info for Jenkins

Jenkins publishes coverage/lcov.info using Coverage Plugin.
Coverage stage only runs on protected branches.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Entire-Checkpoint: 7783c7698334

qt6-after/qt6-migrated-benchmark/MainWindow.h

@@ -2,30 +2,29 @@
 
 #include <QMainWindow>
 #include <QRegularExpression>
-#include <QString>
 
 QT_BEGIN_NAMESPACE
-
-namespace Ui
-{
+namespace Ui {
 class MainWindow;
 }
-
 QT_END_NAMESPACE
 
 class MainWindow : public QMainWindow
 {
     Q_OBJECT
 
-  public:
-    explicit MainWindow(QWidget* parent = nullptr);
-    ~MainWindow() override;
+public:
+    explicit MainWindow(QWidget *parent = nullptr);
+    ~MainWindow();
 
-  private slots:
+    [[nodiscard]] bool validateInput(const QString &text) const;
+    [[nodiscard]] QString getScreenInfo() const;
+
+public slots:
     void onValidateClicked();
     void onScreenInfoClicked();
 
-  private:
-    Ui::MainWindow* ui;
-    QRegularExpression pattern;
+private:
+    Ui::MainWindow *ui;
+    QRegularExpression m_pattern;
 };