fix(co-gahusb): update_task 존재하지 않는 task_id not_found 가드

co-gahusb/tests/test_tasks.py

@@ -48,3 +48,9 @@ async def test_invalid_status_rejected(r):
     tid = (await store.create_task(r, "x", "FE", created_by="Producer"))["task_id"]
     with pytest.raises(ValueError):
         await store.update_task(r, tid, "bogus", "FE")
+
+
+async def test_update_nonexistent_task_returns_not_found(r):
+    res = await store.update_task(r, 999, "done", "FE")
+    assert res["ok"] is False
+    assert res["error"] == "not_found"