tests/cli(fix): Include stderr in output capture for CLI tests

why: Some CLI output goes to stderr and was being missed
what:
- Capture both stdout and stderr in test_sync_cli_filter_non_existent
- Ensures complete output validation in test

Author: tony

tests/test_cli.py

@@ -103,7 +103,8 @@ def test_sync_cli_filter_non_existent(
     with contextlib.suppress(SystemExit):
         cli(["sync", *sync_args])
 
-    output = "".join(list(caplog.messages) + list(capsys.readouterr().out))
+    captured = capsys.readouterr()
+    output = "".join([*caplog.messages, captured.out, captured.err])
 
     if expected_in_out is not None:
         if isinstance(expected_in_out, str):