## 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>
### What problem does this PR solve?
`DeepLParam.check()` validated `self.top_n`, but DeepL has no such
parameter (it is not defined on the param class or its base), so
`check()` always raised `AttributeError` and a DeepL component could
never pass validation. Removed the bogus `top_n` check.
Also fixed the `_run` except branch, which computed
`be_output("**Error**...")` but never returned it, silently dropping the
error message.
Closes#16329
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Add test cases
### Testing
Added `test/unit_test/agent/component/test_deepl.py` covering
`DeepLParam.check()` with valid defaults and rejection of invalid
source/target languages.
