fix(postgres): handle missing checkpoint_id in _get_channel_writes_history; update test signatures

Two fixes exposed by running the postgres test suite against a local
postgres instance:

1. `PostgresSaver._get_channel_writes_history` /
   `AsyncPostgresSaver._aget_channel_writes_history` required
   `checkpoint_id` in the passed config, raising `KeyError` when called
   with just `thread_id` (e.g. `graph.aget_state({"thread_id": "..."})`).
   Now resolves to the latest checkpoint via `get_tuple`/`aget_tuple`
   when the id is missing.

2. `test_get_checkpoint_no_channel_values` (sync + async) monkeypatched
   `_load_checkpoint_tuple` with the old `(value, cur)` signature. Method
   now takes `(value)` only since delta reconstruction moved out of the
   tuple-load path — updated both tests.

Local postgres (`brew install pgvector postgresql@16`, running on port
5441) now exercises all 40 non-vector postgres tests green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

libs/checkpoint-postgres/tests/test_sync.py

@@ -348,9 +348,9 @@ def test_get_checkpoint_no_channel_values(
 
         load_checkpoint_tuple = saver._load_checkpoint_tuple
 
-        def patched_load_checkpoint_tuple(value, cur):
+        def patched_load_checkpoint_tuple(value):
             value["checkpoint"].pop("channel_values", None)
-            return load_checkpoint_tuple(value, cur)
+            return load_checkpoint_tuple(value)
 
         monkeypatch.setattr(
             saver, "_load_checkpoint_tuple", patched_load_checkpoint_tuple