chore: Don't add prompt about parent agent when disallow_transfer_to_parent=True

Fixes #844

PiperOrigin-RevId: 764116180

Author: Jacksunwei

src/google/adk/flows/llm_flows/agent_transfer.py

@@ -98,11 +98,11 @@ def _build_target_agents_instructions(
 the function call.
 """
 
-  if agent.parent_agent:
+  if agent.parent_agent and not agent.disallow_transfer_to_parent:
     si += f"""
 Your parent agent is {agent.parent_agent.name}. If neither the other agents nor
 you are best for answering the question according to the descriptions, transfer
-to your parent agent. If you don't have parent agent, try answer by yourself.
+to your parent agent.
 """
   return si