Enhance test submission feature by adding optional hiddenInstructions field for temporary instructions during LLM checks; update API, UI components, and types to support this functionality, improving task evaluation for teachers and challenge authors.

src/pages/tasks/TaskFormPage.tsx

@@ -136,6 +136,7 @@ export const TaskFormPage: React.FC = () => {
         taskId: task.id,
         result: testAnswer.trim(),
         isTest: true,
+        hiddenInstructions: hiddenInstructions.trim() || undefined,
       }).unwrap()
 
       setTestStatus(result.status)