fix(agent): restore be_output and test DeepL error return (#16363)

## Summary

#16332 fixed the missing `return` in DeepL's except branch, but
`ComponentBase.be_output` was removed during the agent refactor (#9113)
while several components still call it. DeepL (and other tools) would
raise `AttributeError` before any error message could be returned.

- Restore `ComponentBase.be_output` as `pd.DataFrame([{"content": v}])`
(same as pre-refactor behavior)
- Add regression test that `_run` returns the `**Error**:` message when
translation fails

Related to #16329

## Test plan

- [x] `test_run_returns_error_on_translation_failure`
- [x] Existing `test_deepl.py` check() tests still pass

---------

Co-authored-by: Harsh Kashyap <harshkashyap@Harshs-MacBook-Pro.local>
Co-authored-by: Zhichang Yu <yuzhichang@gmail.com>

agent/component/base.py

@@ -562,6 +562,10 @@ class ComponentBase(ABC):
             return None
         return self._param.inputs[key].get("value")
 
+    @staticmethod
+    def be_output(v):
+        return pd.DataFrame([{"content": v}])
+
     def get_component_name(self, cpn_id) -> str:
         return self._canvas.get_component(cpn_id)["obj"].component_name.lower()