fix: harden real backend workflows and channel connections

api/src/files/files.service.spec.ts

@@ -27,6 +27,7 @@ describe('FilesService', () => {
         bucket: 'cmpp-platform',
         fileName: '营业执照.png',
         contentType: 'image/png',
+        sizeBytes: '12',
         purpose: 'signature_material',
       }));
 
@@ -47,4 +48,39 @@ describe('FilesService', () => {
       }),
     });
   });
+
+  it('downloads file content from object storage by FileObject id', async () => {
+    const fileObject = {
+      id: 'file-1',
+      tenantId: 'tenant-1',
+      bucket: 'cmpp-platform',
+      objectKey: 'signature-materials/sig-1/file.png',
+      fileName: 'file.png',
+      contentType: 'image/png',
+      sizeBytes: BigInt(12),
+      checksum: null,
+      purpose: 'signature_material',
+      createdAt: new Date('2026-07-06T00:00:00.000Z'),
+    };
+    const prisma = {
+      fileObject: {
+        findUnique: jest.fn().mockResolvedValue(fileObject),
+      },
+    };
+    const objectStorage = {
+      getObject: jest.fn().mockResolvedValue(Buffer.from('file-content')),
+    };
+    const service = new FilesService(prisma as never, objectStorage as never);
+
+    await expect(service.getDownload('file-1')).resolves.toEqual({
+      fileObject: expect.objectContaining({
+        id: 'file-1',
+        fileName: 'file.png',
+        sizeBytes: '12',
+      }),
+      content: Buffer.from('file-content'),
+    });
+    expect(prisma.fileObject.findUnique).toHaveBeenCalledWith({ where: { id: 'file-1' } });
+    expect(objectStorage.getObject).toHaveBeenCalledWith('signature-materials/sig-1/file.png');
+  });
 });