Improve BackgroundService destructor to avoid unnecessary cancel calls

Previously, BackgroundService.__del__() would always call cancel() regardless
of whether the service was still running. This caused missleading
messages suggesting services weren't properly stopped.

Services should always be explicitly stopped before deletion,
so destructor now only calls cancel() if the service is still running.
The error log can help identify and clean up such cases.

Signed-off-by: Elzbieta Kotulska <elzbieta.kotulska@frequenz.com>

Author: ela-kotulska-frequenz

src/frequenz/sdk/actor/_background_service.py

@@ -278,7 +278,9 @@ def __del__(self) -> None:
 
         Cancel all running tasks spawned by this background service.
         """
-        self.cancel("{self!r} was deleted")
+        if self.is_running:
+            _logger.error("%s was not stopped, before delete", repr(self))
+            self.cancel(f"{self!r} was deleted")
 
     def __repr__(self) -> str:
         """Return a string representation of this instance.