fix(langgraph): CheckpointTask.state can be a StateSnapshot (#6201)

This adds `StateSnapshot` to the union type annotation of
`CheckpointTask.state`.

The annotation was previously incomplete: `map_debug_checkpoint()`
generates `CheckpointPayload` objects from `PregelTask` objects, and the
`state` field in `PregelTask` is of type `None | RunnableConfig |
StateSnapshot`.

libs/langgraph/langgraph/pregel/debug.py

@@ -48,7 +48,7 @@ class CheckpointTask(TypedDict):
     name: str
     error: str | None
     interrupts: list[dict]
-    state: RunnableConfig | None
+    state: StateSnapshot | RunnableConfig | None
 
 
 class CheckpointPayload(TypedDict):