feat: use normalized checkbox values in field handlers

src/Form/Service/Abstract/AbstractParticipantFieldHandler.php

@@ -153,6 +153,21 @@ abstract class AbstractParticipantFieldHandler implements ParticipantFieldHandle
         return is_numeric($value) ? (int) $value : null;
     }
 
+    /**
+     * Normalizes a checkbox/form toggle value to a boolean.
+     *
+     * Form checkboxes may submit true, false, 1, 0, '1', '0', or be absent.
+     * This method treats only true, 1, and '1' as checked; everything else as unchecked.
+     *
+     * @param mixed $value The raw value from form submission
+     *
+     * @return bool True if the checkbox is considered checked, false otherwise
+     */
+    protected function normalizeCheckboxValue(mixed $value): bool
+    {
+        return true === $value || 1 === $value || '1' === $value;
+    }
+
     /**
      * Returns field state modifications that should be applied after processing.
      *