fix: build_graph is not awaited.

Fixes #927

PiperOrigin-RevId: 764113488

Author: Jacksunwei

src/google/adk/cli/agent_graph.py

@@ -130,7 +130,7 @@ def draw_edge(from_name, to_name):
 
   draw_node(agent)
   for sub_agent in agent.sub_agents:
-    build_graph(graph, sub_agent, highlight_pairs)
+    await build_graph(graph, sub_agent, highlight_pairs)
     draw_edge(agent.name, sub_agent.name)
   if isinstance(agent, LlmAgent):
     for tool in await agent.canonical_tools():