[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/getting_started/observability/workflow_observability.py

@@ -6,7 +6,7 @@ from agent_framework import (
     Executor,
     WorkflowBuilder,
     WorkflowContext,
-    WorkflowOutputEvent,
+    
     handler,
 )
 from agent_framework.observability import configure_otel_providers, get_tracer
@@ -93,7 +93,7 @@ async def run_sequential_workflow() -> None:
 
     output_event = None
     async for event in workflow.run("Hello world", stream=True):
-        if isinstance(event, WorkflowOutputEvent):
+        if event.type == "output":
             # The WorkflowOutputEvent contains the final result.
             output_event = event