Check whether action handler has source in collect_actions

Author: Aksem

src/finecode/config/collect_actions.py

@@ -41,7 +41,9 @@ def _collect_action_handler_configs_in_config(
     action_handlers_configs = config["tool"]["finecode"].get("action_handler", [])
     action_handler_config_by_source: dict[str, dict[str, Any]] = {}
     for handler_def in action_handlers_configs:
-        # TODO: validate that source  field exist?
+        if "source" not in handler_def or not isinstance(handler_def['source'], str):
+            raise config_models.ConfigurationError("Action handler definition expected to have a 'source' field(to identify handler) and it should be a string")
+
         handler_config = handler_def.get("config", None)
         if handler_config is not None:
             action_handler_config_by_source[handler_def["source"]] = handler_config