fix(graph): prevent reset_executor_state from corrupting MultiAgentBase state

[giulio-leone]

Summary

Fixes #1775 — GraphNode.reset_executor_state() corrupts MultiAgentBase executor state by overwriting GraphState with AgentState.

Root Cause

reset_executor_state() checks hasattr(self.executor, 'state') without verifying the state type:

# Before (broken)
if hasattr(self.executor, 'state'):
    self.executor.state = AgentState(self._initial_state.get())  # Overwrites GraphState!

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

Impact

Two call sites affected:

Call site	Trigger
_execute_node	reset_on_revisit=True with nested graph in a cycle
deserialize_state	Always — iterates all nodes when restoring a completed run

Fix

Mirror the __post_init__ guard:

# After (fixed)
if hasattr(self.executor, 'state') and hasattr(self.executor.state, 'get'):
    self.executor.state = AgentState(self._initial_state.get())

Test

Added test_reset_executor_state_preserves_graph_state_for_nested_graph — creates a GraphNode with a nested Graph executor and verifies reset_executor_state() preserves the GraphState type.

All 49 graph tests pass.

[giulio-leone]

Friendly ping — prevents reset_executor_state from corrupting MultiAgentBase state by preserving agent-level state that should persist across executor resets.