Fix AsyncPostgresStore._cursor row type (#4623)

`AsyncPostgresStore._cursor` is expected to yield
`AsyncCursor[DictRow]`. In the `else` branch the `row_factory` was not
defined, meaning that it would yield `AsyncCursor[TupleRow]` (default).
This resulted in an error in
[langgraph/store/postgres/aio.py#L249](https://github.com/langchain-ai/langgraph/blob/main/libs/checkpoint-postgres/langgraph/store/postgres/aio.py#L249)
(`TypeError: tuple indices must be integers or slices, not str`).

libs/checkpoint-postgres/langgraph/store/postgres/aio.py

@@ -558,6 +558,6 @@ class AsyncPostgresStore(AsyncBatchedBaseStore, BasePostgresStore[_ainternal.Con
             else:
                 async with (
                     self.lock,
-                    conn.cursor(binary=True) as cur,
+                    conn.cursor(binary=True, row_factory=dict_row) as cur,
                 ):
                     yield cur