Update ADK doc according to issue #1173 - 9 - AgentTool include_plugins

[adk-bot]

9. Summary of the change

Doc file: docs/agents/multi-agents.md

Current state:

The documentation for AgentTool does not mention the include_plugins parameter.

Proposed Change:

Add a note or update the AgentTool example to mention the include_plugins parameter.
Explain that include_plugins (default: True) controls whether the child agent inherits plugins from the parent runner. Setting it to False allows for isolated execution.

Reasoning:
New parameter include_plugins added to AgentTool allows controlling plugin propagation in multi-agent setups. This is important for configuring agent isolation.

Reference: src/google/adk/tools/agent_tool.py

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