[BREAKING] Python: Refactor workflow events to unified discriminated union pattern (#3690)

* Refactor events

* Merge main

* Fixes

* Cleanup

* Update samples and tests

* Remove unused imports

* PR feedback

* Merge main. Add properties for events to help typing

* Formatting

* Cleanup

* use builtins.type to avoid shadowing by WorkflowEvent.type attribute

* Final improvements

python/samples/semantic-kernel-migration/processes/nested_process.py

@@ -17,7 +17,7 @@ from agent_framework import (
     WorkflowBuilder,
     WorkflowContext,
     WorkflowExecutor,
-    WorkflowOutputEvent,
+    
     handler,
 )
 from pydantic import BaseModel, Field
@@ -257,7 +257,7 @@ async def run_agent_framework_nested_workflow(initial_message: str) -> Sequence[
 
     results: list[str] = []
     async for event in outer_workflow.run(initial_message, stream=True):
-        if isinstance(event, WorkflowOutputEvent):
+        if event.type == "output":
             results.append(cast(str, event.data))
 
     return results