feat(channels): step-based eager snapshot_frequency via _DeltaSnapshot ext type

Replaces the write-count (_write_count) snapshot mechanism with a clean
step-based approach: pregel's create_checkpoint fires snapshots every N
pregel steps regardless of whether the channel was written (eager).

Key changes:
- snapshot_frequency=None (default) for pure delta; int N for snapshot every N steps
- DeltaChannel.checkpoint() always returns DELTA_SENTINEL; snapshot logic
  lives in create_checkpoint which has the step number
- create_checkpoint bumps channel version via get_next_version when the
  channel wasn't written at a snapshot step (eager: always stores the blob)
- _DeltaSnapshot NamedTuple registered as EXT_DELTA_SNAPSHOT (code 7) in
  the msgpack serde — no dict key collision, type tag does the dispatch
- InMemorySaver and PostgresSaver _get_channel_writes_history updated:
  _DeltaSnapshot blobs collect pending_writes before terminating (they
  encode the NEXT step's transition, not subsumed by the snapshot unlike
  pre-delta migration blobs)

Tests confirm:
- Snapshots fire at every N steps even when channel has no write that step
- Correct accumulated state after reconstruction from snapshot + replay

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

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

@@ -16,7 +16,7 @@ from langgraph.checkpoint.base import (
     _ChannelWritesHistory,
     get_checkpoint_id,
 )
-from langgraph.checkpoint.serde.types import TASKS
+from langgraph.checkpoint.serde.types import TASKS, _DeltaSnapshot
 from psycopg.types.json import Jsonb
 
 MetadataInput = dict[str, Any] | None
@@ -281,13 +281,24 @@ class BasePostgresSaver(BaseCheckpointSaver[str]):
 
         collected: list[PendingWrite] = []  # newest first; reversed at the end
         for cid in ancestors:
-            # Pre-delta blob terminator: subsumes any writes at this ancestor.
             ver = ver_of.get(cid)
             if ver is not None:
                 seed_blob = blob_by_ver.get(ver)
                 if seed_blob is not None and seed_blob[0] != "empty":
                     blob_value = self.serde.loads_typed(seed_blob)
                     if blob_value is not DELTA_SENTINEL:
+                        if isinstance(blob_value, _DeltaSnapshot):
+                            # Step-based snapshot: collect this ancestor's
+                            # pending_writes first (they encode the NEXT step's
+                            # transition, not subsumed by the snapshot blob).
+                            for type_tag, write_blob, task_id, _idx in writes_by_cid.get(
+                                cid, []
+                            ):
+                                val = self.serde.loads_typed((type_tag, write_blob))
+                                collected.append((task_id, channel, val))
+                            collected.reverse()
+                            return _ChannelWritesHistory(seed=blob_value, writes=collected)
+                        # Pre-delta blob: subsumes this ancestor's writes.
                         collected.reverse()
                         return _ChannelWritesHistory(seed=blob_value, writes=collected)
             for type_tag, write_blob, task_id, _idx in writes_by_cid.get(cid, []):