Build a Multi-Agent Marathon Planner with ADK and A2A

In this codelab, you build a multi-agent system where three specialized AI agents collaborate to plan a city marathon. A Marathon Planner agent orchestrates the design, an Evaluator agent scores plan quality using Gemini Entreprise Agent Platform Evaluation, and a Simulation Controller agent validates readiness — all communicating through the Agent-to-Agent (A2A) protocol.