fix: resolve mypy type errors for langgraph ToolNode fallback

- Use class-based fallback instead of assignment for ToolNode in TYPE_CHECKING block
- Add type: ignore for no-redef to handle the intentional redefinition
- This fixes compatibility with Python 3.9 mypy type checking

Author: ryoppippi

stackone_ai/integrations/langgraph.py

@@ -24,7 +24,9 @@
     try:
         from langgraph.prebuilt import ToolNode
     except Exception:  # pragma: no cover
-        ToolNode = Any
+
+        class ToolNode:  # type: ignore[no-redef]
+            pass
 
 
 def _ensure_langgraph() -> None: