docs: updated built-in tools to use consistent AgentTool call format across files

[Shubhamsaboo]

Updated the built-in tools section to match how we first introduce the AgentTool concept in the Agent-as-a-tool section.

This keeps the usage consistent with the example shown in the ADK docs here: https://google.github.io/adk-docs/tools/function-tools/#3-agent-as-a-tool

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