feat(docs): warn that StateGraph is a builder class (#6417)

Author: mdrxy

libs/langgraph/langgraph/graph/state.py

@@ -117,6 +117,13 @@ class StateGraph(Generic[StateT, ContextT, InputT, OutputT]):
     will be used to aggregate the values of that key received from multiple nodes.
     The signature of a reducer function is `(Value, Value) -> Value`.
 
+    !!! warning
+
+        `StateGraph` is a builder class and cannot be used directly for execution.
+        You must first call `.compile()` to create an executable graph that supports
+        methods like `invoke()`, `stream()`, `astream()`, and `ainvoke()`. See the
+        `CompiledStateGraph` documentation for more details.
+
     Args:
         state_schema: The schema class that defines the state.
         context_schema: The schema class that defines the runtime context.