A2A support in ADK JS

[kalenkevich]

** Please make sure you read the contribution guide and file the issues in the
right place. **
Contribution guide.

Is your feature request related to a problem? Please describe.
Bring adk-js into full compliance with the latest A2A protocol specification. This will allow developers using adk-js to easily expose their multi-agent systems via an A2A Server, or connect to remote external agents acting as an A2A Client

A2A Integration Development Plan - ADK-JS

This document outlines the development plan for A2A (Agent-To-Agent) protocol integration in adk-js.

📋 Overview

The A2A integration enables two interaction models:

1. Hosting (Server-Side): A2AAgentExecutor allows ADK-JS agents to be hosted as A2A-compliant agents.
2. Delegation (Client-Side): A2ARemoteAgent allows an ADK agent tree to delegate tasks to external A2A agents.

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🗺️ Phased Roadmap

🟢 Phase 1: Foundations & Type Converters

• Objective: Establish core translation utilities for data exchange.
• Completed Work: A2A integration: Add util to convert A2A <-> GenAI parts. #160 A2A integration: add utils to convert A2A <-> ADK events #165 A2A integration: Fix event conversion issues to align with A2A <-> ADK Go implementation #178

🟢 Phase 2: Execution Context

• Objective: Manage state and context for A2A execution threads.
• Completed Work: A2A integration: add executor context #166

🟢 Phase 3: Agent Card

• Objective: Translate ADK Agent definitions into A2A formats.
• Completed Work: A2A integration: Add agent card #187

🟡 Phase 4: Server-Side Hosting (Agent Executor)

• Objective: Create the core hosting layer for executing agents.
• Completed Work: A2A integration: Agent executor #186
  • Streaming response assembly.
• Current Work: Finalizing unit/integration tests and resolving type alignment issues.

🔵 Phase 5: Client-Side Delegation (Remote Agent)

• Objective: Support external A2A agents as nodes in an ADK tree.
• Current Work: - Related: A2A support in ADK JS #189

🔵 Phase 6: E2E test

• Objective: Add E2E tests to cover A2A integration in ADK