test(update): strengthen UnicodeDecodeError regression to assert_not_called()

Follow-up per review of #74631.

The prior assertion (call_count == 0 OR interactive != True) also
passed if an unintended non-interactive migration occurred, which the
safe fallback (response='n') is supposed to prevent entirely. Replaced
with mock_migrate.assert_not_called().

6/6 pass in the full tests/hermes_cli/test_update_yes_flag.py file.

tests/hermes_cli/test_update_yes_flag.py

@@ -181,9 +181,7 @@ class TestUnicodeDecodeErrorInUpdatePrompts:
 
         out = capsys.readouterr().out
         assert "hermes config migrate" in out
-        assert mock_migrate.call_count == 0 or (
-            mock_migrate.call_args and mock_migrate.call_args.kwargs.get("interactive") is not True
-        )
+        mock_migrate.assert_not_called()
 
     def test_stash_restore_unicode_decode_error_falls_through_to_skip(self, tmp_path, capsys):
         from hermes_cli.update_cmd import _restore_stashed_changes