test(langgraph): compare read-back checkpoints in the immutability saver

MemorySaverAssertImmutable recorded the checkpoint object handed to put,
then compared it against one read back through get. Those two are not the
same shape: channel_values are stored per (channel, version), so a channel
a step did not write is refilled from the blob its inherited version still
points at.

Every channel except DeltaChannel writes its value into channel_values on
every checkpoint, so the two agreed by accident. A DeltaChannel stores
nothing except at a snapshot, so once one snapshots and a later step does
not write it, the saver reports a checkpoint that changed after it was
written when nothing was mutated.

Reproducible on main with no fork involved: a delta channel with
snapshot_frequency=1 written by the first node and left alone by the next
two trips the assertion. Existing delta tests miss it only because they
all use snapshot_frequency=1000.

Record what the saver reads back instead. Comparing read-back against
read-back still catches a checkpoint whose stored data really changed.

libs/langgraph/tests/memory_assert.py

@@ -86,11 +86,19 @@ class MemorySaverAssertImmutable(InMemorySaver):
                 )
                 == saved
             ), config["configurable"]["checkpoint_ns"]
-        self.storage_for_copies[thread_id][checkpoint_ns][checkpoint["id"]] = (
-            self.serde.dumps_typed(checkpoint)
-        )
         # call super to write checkpoint
-        return super().put(config, checkpoint, metadata, new_versions)
+        next_config = super().put(config, checkpoint, metadata, new_versions)
+        # Record the checkpoint as the saver reads it back, not the object it
+        # was handed. `channel_values` are stored per (channel, version), so a
+        # channel a step did not write is refilled from the blob its inherited
+        # version still points at. A `DeltaChannel` omits its value except at a
+        # snapshot, which makes the two representations differ for reasons that
+        # are not mutation. Comparing read-back against read-back still catches
+        # a checkpoint whose stored data actually changed.
+        self.storage_for_copies[thread_id][checkpoint_ns][checkpoint["id"]] = (
+            self.serde.dumps_typed(super().get(next_config))
+        )
+        return next_config
 
 
 class MemorySaverNoPending(InMemorySaver):