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/en/concepts/event-listener.mdx

@@ -256,6 +256,7 @@ CrewAI provides a wide range of events that you can listen for:
 - **FlowCreatedEvent**: Emitted when a Flow is created
 - **FlowStartedEvent**: Emitted when a Flow starts execution
 - **FlowFinishedEvent**: Emitted when a Flow completes execution
+- **FlowFailedEvent**: Emitted when a Flow execution fails. Contains the flow name and the exception that ended the execution.
 - **FlowPausedEvent**: Emitted when a Flow is paused waiting for human feedback. Contains the flow name, flow ID, method name, current state, message shown when requesting feedback, and optional list of possible outcomes for routing.
 - **FlowPlotEvent**: Emitted when a Flow is plotted
 - **MethodExecutionStartedEvent**: Emitted when a Flow method starts execution