fix serialization issue with custom service task, w/ essweine (#605)

Co-authored-by: burnettk <burnettk@users.noreply.github.com>

spiffworkflow-backend/bin/execute_tasks_for_process_instance.py

@@ -7,7 +7,7 @@ def main() -> None:
     app = create_app()
     with app.app_context():
         execution_strategy_name = app.config["SPIFFWORKFLOW_BACKEND_ENGINE_STEP_DEFAULT_STRATEGY_BACKGROUND"]
-        process_instance = ProcessInstanceModel.query.filter_by(id=29).first()
+        process_instance = ProcessInstanceModel.query.filter_by(id=2).first()
         ProcessInstanceService.run_process_instance_with_processor(
             process_instance, execution_strategy_name=execution_strategy_name
         )

spiffworkflow-backend/src/spiffworkflow_backend/services/process_instance_processor.py

@@ -41,6 +41,7 @@ from SpiffWorkflow.serializer.exceptions import MissingSpecError  # type: ignore
 from SpiffWorkflow.spiff.parser.process import SpiffBpmnParser  # type: ignore
 from SpiffWorkflow.spiff.serializer.config import SPIFF_CONFIG  # type: ignore
 from SpiffWorkflow.spiff.serializer.task_spec import ServiceTaskConverter  # type: ignore
+from SpiffWorkflow.spiff.specs.defaults import ServiceTask  # type: ignore
 from SpiffWorkflow.task import Task as SpiffTask  # type: ignore
 from SpiffWorkflow.util.deep_merge import DeepMerge  # type: ignore
 from SpiffWorkflow.util.task import TaskIterator  # type: ignore
@@ -99,12 +100,21 @@ from spiffworkflow_backend.services.workflow_execution_service import execution_
 from spiffworkflow_backend.specs.start_event import StartEvent
 from sqlalchemy import and_
 
+
+# this custom converter is just so we use 'ServiceTask' as the typename in the serialization
+# rather than 'CustomServiceTask'
+class CustomServiceTaskConverter(ServiceTaskConverter):  # type: ignore
+    def __init__(self, target_class, registry, typename: str = "ServiceTask"):  # type: ignore
+        super().__init__(target_class, registry, typename)
+
+
 SPIFF_CONFIG[StartEvent] = EventConverter
 SPIFF_CONFIG[JSONDataStore] = JSONDataStoreConverter
 SPIFF_CONFIG[JSONFileDataStore] = JSONFileDataStoreConverter
 SPIFF_CONFIG[KKVDataStore] = KKVDataStoreConverter
 SPIFF_CONFIG[TypeaheadDataStore] = TypeaheadDataStoreConverter
-SPIFF_CONFIG[CustomServiceTask] = ServiceTaskConverter
+SPIFF_CONFIG[CustomServiceTask] = CustomServiceTaskConverter
+del SPIFF_CONFIG[ServiceTask]
 
 # Sorry about all this crap.  I wanted to move this thing to another file, but
 # importing a bunch of types causes circular imports.