chore(deps): widen langgraph-checkpoint upper bound to <5.0.0

Allow langgraph-checkpoint >=4.0.0 which contains a security fix for
CVE-2026-27794 (pickle deserialization in BaseCache). This library uses
a custom JsonPlusRedisSerializer that uses orjson/msgpack and never
pickle, so the breaking change (pickle_fallback=False default) has no
impact.

Closes #152

Author: bsbodden

poetry.lock

@@ -18,7 +18,7 @@ packages = [{ include = "langgraph" }]
 
 [tool.poetry.dependencies]
 python = ">=3.10,<3.14"
-langgraph-checkpoint = ">=3.0.0,<4.0.0"
+langgraph-checkpoint = ">=3.0.0,<5.0.0"
 redisvl = ">=0.11.0,<1.0.0"
 redis = ">=5.2.1"
 orjson = "^3.9.0"