Need to disable Agent Tool Parallel Behaviour

Sub‑agents in ADK are currently not functioning efficiently. The main issue is that sub‑agents are unable to return their output back to the manager/orchestrator—instead, they respond directly to the end user. As a workaround, I converted the agent into a tool, which works better for now.

However, in certain scenarios I need to disable parallel execution for tools. This option is currently not supported for AgentTool.

**Request**
Please add support for disabling parallel execution in AgentTool.