feat: Implement durability mode argument (#5432)

- Replaces checkpoint_during: bool
- checkpoint_during is deprecated but still respected
- We implement three durability modes (from least to most durable):
- "exit" - save checkpoint only when the graph exits (equivalent to
checkpoint_during=False)
- "async" - save checkpoint asynchronously while the next step executes
(the default, equivalent to old checkpoint_during=True)
- "sync" - save checkpoint synchronously before the next step starts
(new mode, slower but most durable)

Co-authored-by: Sydney Runkle <54324534+sydney-runkle@users.noreply.github.com>

libs/checkpoint-postgres/langgraph/checkpoint/postgres/shallow.py

@@ -191,7 +191,7 @@ class ShallowPostgresSaver(BasePostgresSaver):
     ) -> None:
         warnings.warn(
             "ShallowPostgresSaver is deprecated as of version 2.0.20 and will be removed in 3.0.0. "
-            "Use PostgresSaver instead, and invoke the graph with `graph.invoke(..., checkpoint_during=False)`.",
+            "Use PostgresSaver instead, and invoke the graph with `graph.invoke(..., durability='exit')`.",
             DeprecationWarning,
             stacklevel=2,
         )
@@ -547,7 +547,7 @@ class AsyncShallowPostgresSaver(BasePostgresSaver):
     ) -> None:
         warnings.warn(
             "AsyncShallowPostgresSaver is deprecated as of version 2.0.20 and will be removed in 3.0.0. "
-            "Use AsyncPostgresSaver instead, and invoke the graph with `await graph.ainvoke(..., checkpoint_during=False)`.",
+            "Use AsyncPostgresSaver instead, and invoke the graph with `await graph.ainvoke(..., durability='exit')`.",
             DeprecationWarning,
             stacklevel=2,
         )