docs(open-8974): add agent-as-a-tool example to ADK tracing notebook

Add Example 4 demonstrating the AgentTool + plain Python tool pattern:
- Orchestrator with get_current_time (direct function tool)
- WeatherAgent and CalculatorAgent wrapped via AgentTool
- Query that exercises all three tool types
- Trace hierarchy diagram showing how Openlayer surfaces inner tools

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: viniciusdsmello