Capture checkpoint events as a drainable stream.
Surfaces stream_mode="checkpoints" data on run.checkpoints as
a StreamChannel[dict[str, Any]]. Each item is in the same format
as returned by get_state().
Checkpoint events are only emitted when a checkpointer is configured on the graph. When no checkpointer is present, the projection exists but receives no events.
Only events at the run's own scope are captured; checkpoint data from
deeper subgraphs is available on the respective subgraph handle's
.checkpoints projection.
Native transformer — run.checkpoints is a direct attribute.
