feat(form-builder): FormFieldValidationRuleService + legacy backfill + snapshot + library row-copy

api/tests/Feature/FormBuilder/Public/PublicFormSeederTest.php

@@ -187,7 +187,11 @@ final class PublicFormSeederTest extends TestCase
         $this->assertArrayHasKey('sectie_voorkeur', $fields);
         $this->assertSame(FormFieldType::SECTION_PRIORITY->value, $fields['sectie_voorkeur']['field_type']);
         $this->assertSame('full', $fields['sectie_voorkeur']['display_width']);
-        $this->assertSame(['max_priorities' => 3], $fields['sectie_voorkeur']['validation_rules']);
+        // WS-5b canonicalised `max_priorities` → `max_selected` (shared
+        // semantic of "cap on entries in a list-valued field"). Resource
+        // emits the new canonical key via
+        // FormFieldValidationRuleService::toJsonShape().
+        $this->assertSame(['max_selected' => 3], $fields['sectie_voorkeur']['validation_rules']);
     }
 
     public function test_time_slots_endpoint_returns_at_least_four_volunteer_rows(): void