Fix semantics of put_writes/list (#1436)

* Fix semantics of put_writes/list

- put_writes(error) should not prevent saving future successful if task is retried successfully
- put_writes(writes) should be a no-op if non-error writes already exist for that task (this prevents tasks executed more than once from modifying writes previously saved / acted on)
- checkpoints should not include channel default values (ie those without a version)
- list() should fetch and return writes for each checkpoint

* Lint

* Rm print

* Fix import

* Lint

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

@@ -150,6 +150,7 @@ class PostgresSaver(BasePostgresSaver):
                 }
                 if value["parent_checkpoint_id"]
                 else None,
+                self._load_writes(value["pending_writes"]),
             )
 
     def get_tuple(self, config: RunnableConfig) -> Optional[CheckpointTuple]:
@@ -317,16 +318,6 @@ class PostgresSaver(BasePostgresSaver):
             task_id (str): Identifier for the task creating the writes.
         """
         with self._cursor(pipeline=True) as cur:
-            cur.execute(
-                self.DELETE_WRITES_SQL,
-                (
-                    config["configurable"]["thread_id"],
-                    config["configurable"]["checkpoint_ns"],
-                    config["configurable"]["checkpoint_id"],
-                    task_id,
-                    len(writes),
-                ),
-            )
             cur.executemany(
                 self.UPSERT_CHECKPOINT_WRITES_SQL,
                 self._dump_writes(