fix(task): tolerate invalid default config in usage cache

backend/packages/harness/deerflow/tools/builtins/task_tool.py

@@ -35,7 +35,7 @@ def _token_usage_cache_enabled(app_config: "AppConfig | None") -> bool:
     if app_config is None:
         try:
             app_config = get_app_config()
-        except FileNotFoundError:
+        except (FileNotFoundError, ValueError):
             return False
     return bool(getattr(getattr(app_config, "token_usage", None), "enabled", False))
 

backend/tests/test_task_tool_core_logic.py

@@ -1214,11 +1214,12 @@ def test_terminal_event_usage_none_when_no_records(monkeypatch):
     assert completed[0]["usage"] is None
 
 
-def test_subagent_usage_cache_is_skipped_when_config_file_is_missing(monkeypatch):
+@pytest.mark.parametrize("error", [FileNotFoundError("missing config"), ValueError("invalid config")])
+def test_subagent_usage_cache_is_skipped_when_default_config_cannot_load(monkeypatch, error):
     monkeypatch.setattr(
         task_tool_module,
         "get_app_config",
-        MagicMock(side_effect=FileNotFoundError("missing config")),
+        MagicMock(side_effect=error),
     )
 
     assert task_tool_module._token_usage_cache_enabled(None) is False