fix(graph): prevent reset_executor_state from corrupting MultiAgentBase state

GraphNode.reset_executor_state() checked hasattr(self.executor, 'state')
but did not verify the state type before overwriting it with AgentState.
When the executor is a MultiAgentBase (e.g. a nested Graph), its state
is a GraphState, and overwriting it with AgentState corrupts the
executor.

The __post_init__ method already had the correct guard:
  hasattr(self.executor.state, 'get')
but reset_executor_state() was missing it.

This affected two call sites:
- _execute_node (when reset_on_revisit is enabled with nested graphs)
- deserialize_state (unconditionally resets all nodes on completed runs)

Fixes #1775

Author: giulio-leone

src/strands/multiagent/graph.py

@@ -188,7 +188,7 @@ def reset_executor_state(self) -> None:
         if hasattr(self.executor, "messages"):
             self.executor.messages = copy.deepcopy(self._initial_messages)
 
-        if hasattr(self.executor, "state"):
+        if hasattr(self.executor, "state") and hasattr(self.executor.state, "get"):
             self.executor.state = AgentState(self._initial_state.get())
 
         # Reset execution status

tests/strands/multiagent/test_graph.py

@@ -2405,3 +2405,37 @@ async def stream_async(self, prompt=None, **kwargs):
     assert result.completed_nodes == 2
     assert "custom_node" in result.results
     assert "regular_node" in result.results
+
+
+@pytest.mark.asyncio
+async def test_reset_executor_state_preserves_graph_state_for_nested_graph():
+    """Verify reset_executor_state does not corrupt MultiAgentBase state.
+
+    When a GraphNode wraps a MultiAgentBase executor (e.g. a nested Graph),
+    reset_executor_state() must not overwrite GraphState with AgentState.
+    Regression test for #1775.
+    """
+    inner_agent = create_mock_agent("inner", "inner response")
+    inner_builder = GraphBuilder()
+    inner_builder.add_node(inner_agent, "inner_node")
+    inner_builder.set_entry_point("inner_node")
+    inner_graph = inner_builder.build()
+
+    # inner_graph.state is a GraphState, not AgentState
+    assert isinstance(inner_graph.state, GraphState)
+
+    node = GraphNode(node_id="nested", executor=inner_graph)
+
+    # Simulate a completed execution
+    node.execution_status = Status.COMPLETED
+    node.result = NodeResult(result=MagicMock(), status=Status.COMPLETED)
+
+    # Reset should NOT corrupt the nested graph's state
+    node.reset_executor_state()
+
+    # After reset, the executor's state must still be GraphState
+    assert isinstance(inner_graph.state, GraphState), (
+        "reset_executor_state overwrote GraphState with AgentState"
+    )
+    assert node.execution_status == Status.PENDING
+    assert node.result is None