feat: emit FlowFailedEvent when a flow execution fails

A failed flow never emitted a terminal lifecycle event, so the
`flow_started` scope stayed open and consumers such as tracing closed the
root span with a generic orphaned message instead of the real error.
`kickoff_async` and the resume path now emit `FlowFailedEvent`, paired
with `flow_started` and carrying the exception, after draining pending
handlers and background memory writes. The resume path also emits the
`MethodExecutionStartedEvent` it was missing for the method being resumed,
so its finished or failed event pairs with its own scope instead of
popping the flow's.

docs/edge/ar/concepts/event-listener.mdx

@@ -157,6 +157,7 @@ class MyCustomCrew:
 - **FlowCreatedEvent**: يُرسل عند إنشاء تدفق
 - **FlowStartedEvent**: يُرسل عند بدء تنفيذ تدفق
 - **FlowFinishedEvent**: يُرسل عند اكتمال تنفيذ تدفق
+- **FlowFailedEvent**: يُرسل عند فشل تنفيذ تدفق. يحتوي على اسم التدفق والاستثناء الذي أنهى التنفيذ.
 - **FlowPausedEvent**: يُرسل عند إيقاف تدفق مؤقتًا بانتظار ملاحظات بشرية
 
 ### أحداث LLM