Fix checkboxes reappearing when setting a breakpoint with show_checkboxes OFF

itemChanged fires for any data change (including icon updates in the pause
column), causing on_testChecked to inadvertently restore CheckStateRole via
synchronizeEnabledState. Guard against non-checkbox column changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/testium/main_win/testium_win.py

@@ -553,6 +553,8 @@ class MainWindow(QMainWindow, Ui_MainWindow):
             self.reconnect_signals()
 
     def on_testChecked(self, item, index):
+        if index != self.treeTests.cols['name']['index']:
+            return
         self.checkSelect.setCheckState(Qt.PartiallyChecked)
         self.disconnect_signals()
         try: