fix: handle save_period_to_config errors, remove duplicate _MockIssue

- cli.py: wrap save_period_to_config in try/except to avoid
  unhandled traceback on disk full / permission denial (I2)
- test_cli.py: remove duplicate _MockIssue class definition (I3)

redmine_reporter/cli.py

@@ -492,7 +492,14 @@ def main(argv: Optional[List[str]] = None) -> int:
             from_str = from_date
             to_str = to_date
 
-        save_period_to_config(args.config_path, from_str, to_str, precision, dynamic)
+        try:
+            save_period_to_config(args.config_path, from_str, to_str, precision, dynamic)
+        except Exception as e:
+            print(
+                f"❌ Не удалось сохранить период в конфиг: {e}",
+                file=sys.stderr,
+            )
+            return 1
         print(
             f"📌 Period committed [{from_str} -- {to_str}] → {args.config_path}",
             file=sys.stderr,