fix(checkpoint-postgres,checkpoint-sqlite): normalize Mapping values to dict before JSON serialization

The widened Mapping[str, Any] type on Store.put()/aput() let non-dict
Mappings (e.g. UserDict, MappingProxyType) pass type-checking, but
checkpoint-sqlite and checkpoint-postgres only had a type-checker-only
cast(dict, op.value) rather than an actual runtime conversion, so these
values would raise TypeError from orjson/json during serialization.
InMemoryStore already normalized via dict(op.value); apply the same
normalization here.

Caught by open-swe's automated review on this PR.

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

@@ -7,7 +7,7 @@ import logging
 import re
 import threading
 from collections import defaultdict
-from collections.abc import Callable, Iterable, Iterator, Sequence
+from collections.abc import Callable, Iterable, Iterator, Mapping, Sequence
 from contextlib import contextmanager
 from datetime import datetime
 from typing import (
@@ -354,7 +354,7 @@ class BasePostgresStore(Generic[C]):
                     (
                         _namespace_to_text(op.namespace),
                         op.key,
-                        Jsonb(cast(dict, op.value)),
+                        Jsonb(dict(cast(Mapping[str, Any], op.value))),
                     )
                 )
                 if op.ttl is not None:

libs/checkpoint-sqlite/langgraph/store/sqlite/base.py

@@ -7,7 +7,7 @@ import re
 import sqlite3
 import threading
 from collections import defaultdict
-from collections.abc import Callable, Iterable, Iterator, Sequence
+from collections.abc import Callable, Iterable, Iterator, Mapping, Sequence
 from contextlib import contextmanager
 from typing import Any, Literal, NamedTuple, cast
 
@@ -387,7 +387,7 @@ class BaseSqliteStore:
                     [
                         _namespace_to_text(op.namespace),
                         op.key,
-                        orjson.dumps(cast(dict, op.value)),
+                        orjson.dumps(dict(cast(Mapping[str, Any], op.value))),
                         expires_at,
                         op.ttl,
                     ]