feat: add comprehensive tests for duty schedule import and error handling

- Introduced new tests for the `import_duty_schedule_cmd` to verify behavior when no message or effective user is present, ensuring proper early returns.
- Added tests for admin checks to confirm that only authorized users can initiate duty schedule imports, enhancing security.
- Implemented error handling tests for the `handle_handover_time_text` function to ensure appropriate responses for invalid time formats and non-admin users.
- Enhanced overall test coverage for the duty schedule import functionality, contributing to improved reliability and maintainability of the codebase.
- Updated the `.coverage` file to reflect the latest coverage metrics.

tests/test_handlers_errors.py

@@ -57,6 +57,7 @@ async def test_error_handler_update_none_does_not_crash():
 @pytest.mark.asyncio
 async def test_error_handler_reply_text_raises_does_not_propagate():
     """error_handler: when reply_text raises, exception is caught and logged; does not propagate."""
+
     class FakeUpdate:
         pass
 
@@ -71,7 +72,9 @@ async def test_error_handler_reply_text_raises_does_not_propagate():
 
     with patch("duty_teller.handlers.errors.Update", FakeUpdate):
         with patch("duty_teller.handlers.errors.get_lang", return_value="en"):
-            with patch("duty_teller.handlers.errors.t", return_value="An error occurred."):
+            with patch(
+                "duty_teller.handlers.errors.t", return_value="An error occurred."
+            ):
                 await error_handler(update, context)
     # Handler must not raise; reply_text was attempted
     update.effective_message.reply_text.assert_called_once()