fix(delta-channel): fix chain assembly and get_state paths

- Fix InMemorySaver.get_channel_blob: use correct storage[thread_id][ns]
  nesting and deserialize the checkpoint before extracting channel_versions.
- Pass checkpoint_id to after_checkpoint() in channels_from_checkpoint so
  DeltaChannel seeds _last_checkpoint_id correctly on load; without this
  every turn broke the chain at its boundary.
- Wire _assemble_delta_channels into _prepare_state_snapshot and
  _aprepare_state_snapshot (get_state / get_state_history paths) and into
  perform_superstep / aperform_superstep (update_state paths) — previously
  only the loop __enter__ path did assembly.
- Fix test_get_channel_blob to use the correct storage structure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

libs/checkpoint/tests/test_memory.py

@@ -326,7 +326,9 @@ class TestInMemorySaverDeltaChannel:
         cp = empty_checkpoint()
         cp["id"] = "cp1"
         cp["channel_versions"][channel] = version
-        saver.storage[(thread_id, ns)] = {"cp1": ({}, cp, {})}
+        saver.storage[thread_id][ns] = {
+            "cp1": (serde.dumps_typed(cp), serde.dumps_typed({}), None)
+        }
 
         result = saver.get_channel_blob(thread_id, ns, "cp1", channel)
         assert isinstance(result, DeltaValue)
@@ -336,4 +338,6 @@ class TestInMemorySaverDeltaChannel:
     def test_get_channel_blob_missing(self) -> None:
         """get_channel_blob returns NotImplemented when checkpoint or channel not found."""
         saver = InMemorySaver()
-        assert saver.get_channel_blob("t1", "", "no-such-cp", "messages") is NotImplemented
+        assert (
+            saver.get_channel_blob("t1", "", "no-such-cp", "messages") is NotImplemented
+        )