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)

tests/test_cli.py

@@ -159,17 +159,6 @@ def test_cli_verbose_and_debug_flags_accepted(mock_fetch):
     assert code == 0
 
 
-class _MockIssue:
-    """Простой mock Redmine Issue для CLI-тестов."""
-
-    def __init__(self, issue_id=1, subject="Task", project="Project", status="New"):
-        self.id = issue_id
-        self.subject = subject
-        self.project = project
-        self.status = status
-        self.fixed_version = None
-
-
 @mock.patch.dict(os.environ, {}, clear=True)
 @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
 def test_cli_url_and_api_key_override_env(mock_fetch):