fix(coding-teacher): restore context and read project files

tests/unit/coding-teacher-ui.test.tsx

@@ -93,6 +93,7 @@ describe('teacher side chat', () => {
             sourceCapturedAt: 'now',
             includedSourceMessageIds: [],
             omittedMessages: 0,
+            truncatedMessages: 1,
             status: 'completed',
             response: '教学回答',
           },
@@ -106,6 +107,8 @@ describe('teacher side chat', () => {
     expect(screen.getByLabelText('向老师提问')).toHaveValue('解释代码');
     fireEvent.click(screen.getByRole('button', { name: '提问', exact: true }));
     await screen.findByText('教学回答');
+    expect(screen.getByText('结合当前会话和项目文件答疑')).toBeVisible();
+    expect(screen.getByText('较长的上下文已节选，老师可按需读取原文。')).toBeVisible();
     expect(api.send.mock.calls[0][2].requestId).toBe(api.send.mock.calls[1][2].requestId);
     expect(bringBack).not.toHaveBeenCalled();
     fireEvent.click(screen.getByRole('button', { name: '带回主会话草稿' }));