[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/orchestrations/sequential.py

@@ -6,8 +6,9 @@ import asyncio
 from collections.abc import Sequence
 from typing import cast
 
-from agent_framework import ChatMessage, SequentialBuilder, WorkflowOutputEvent
+from agent_framework import ChatMessage
 from agent_framework.azure import AzureOpenAIChatClient
+from agent_framework.orchestrations import SequentialBuilder
 from azure.identity import AzureCliCredential
 from semantic_kernel.agents import Agent, ChatCompletionAgent, SequentialOrchestration
 from semantic_kernel.agents.runtime import InProcessRuntime
@@ -77,7 +78,7 @@ async def run_agent_framework_example(prompt: str) -> list[ChatMessage]:
 
     conversation_outputs: list[list[ChatMessage]] = []
     async for event in workflow.run(prompt, stream=True):
-        if isinstance(event, WorkflowOutputEvent):
+        if event.type == "output":
             conversation_outputs.append(cast(list[ChatMessage], event.data))
 
     return conversation_outputs[-1] if conversation_outputs else []