fix(checkpoint): default InMemorySaver delta history to the latest checkpoint (#9257)

`InMemorySaver.get_delta_channel_history` looked its target up by
`config["configurable"].get("checkpoint_id", "")`, so a config without a
`checkpoint_id` matched no checkpoint, and every channel came back with
no seed and no writes, which a `DeltaChannel` reads as empty. The base
implementation, `PostgresSaver` and `SqliteSaver` read from the thread's
latest checkpoint instead, as `get_tuple` does.

A missing id now resolves to the namespace's newest checkpoint id, with
the same `max` that `InMemorySaver.get_tuple` uses. Graphs always pass a
checkpoint id here, so only direct callers of the saver API were
affected, and a call with an id does the same work as before.

Thanks @Hotragn, who pinned this down on #8242, where @longquanzheng's
branch had already fixed it.

JS twin: langchain-ai/langgraphjs#2979

## Tests

- `test_get_channel_writes_without_checkpoint_id_reads_the_latest` fails
on main (`{'messages': {'writes': []}}`).
- The `checkpoint`, `checkpoint-conformance`, `checkpoint-sqlite`,
`checkpoint-postgres` (Postgres 16), `prebuilt` and `langgraph` suites
pass, the last two without their Redis cases. The DeltaChannel
conformance suite also passes against `InMemorySaver`; CI skips that run
in `libs/checkpoint`, where the conformance package isn't installed, so
the new test lives in `test_memory.py`.

libs/checkpoint/langgraph/checkpoint/memory/__init__.py

@@ -170,8 +170,8 @@ class InMemorySaver(
             return {}
         thread_id = config["configurable"]["thread_id"]
         checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
-        checkpoint_id = config["configurable"].get("checkpoint_id", "")
         ns_storage = self.storage.get(thread_id, {}).get(checkpoint_ns, {})
+        checkpoint_id = get_checkpoint_id(config) or max(ns_storage, default="")
 
         chain: list[str] = []
         target_entry = ns_storage.get(checkpoint_id)

libs/checkpoint/tests/test_memory.py

@@ -420,6 +420,19 @@ class TestInMemorySaverDeltaChannel:
         assert "seed" not in result
         assert result["writes"] == []
 
+    def test_get_channel_writes_without_checkpoint_id_reads_the_latest(self) -> None:
+        saver = InMemorySaver()
+        thread: RunnableConfig = {
+            "configurable": {"thread_id": "t1", "checkpoint_ns": ""}
+        }
+        parent = saver.put(thread, empty_checkpoint(), {}, {})
+        saver.put_writes(parent, [("messages", "hi")], "task1")
+        saver.put(parent, empty_checkpoint(), {}, {})
+
+        result = saver.get_delta_channel_history(config=thread, channels=["messages"])
+
+        assert result == {"messages": {"writes": [("task1", "messages", "hi")]}}
+
 
 class TestBaseFallbackGetChannelWrites:
     """Exercises the `BaseCheckpointSaver.get_delta_channel_history` default