Add A2A (Agent-to-Agent) protocol support

[minorun365]

Summary

This PR adds support for the A2A (Agent-to-Agent) protocol, enabling agents to communicate with each other using the JSON-RPC 2.0 protocol.

New Features

• BedrockAgentCoreA2AApp: A Starlette-based application class for hosting A2A agents
• AgentCard & AgentSkill: Models for describing agent capabilities
• @entrypoint decorator: For defining message handlers
• JSON-RPC 2.0 support: Standard A2A protocol implementation on port 9000

New Files

• src/bedrock_agentcore/runtime/a2a_app.py - Main A2A application class
• src/bedrock_agentcore/runtime/a2a_models.py - A2A data models
• tests/bedrock_agentcore/runtime/test_a2a_app.py - Unit tests for A2A app
• tests/bedrock_agentcore/runtime/test_a2a_models.py - Unit tests for A2A models

Usage Example

from bedrock_agentcore.runtime import AgentCard, AgentSkill, BedrockAgentCoreA2AApp

agent_card = AgentCard(
    name="My Agent",
    description="A helpful assistant",
    version="1.0.0",
    skills=[AgentSkill(id="qa", name="Q&A", description="Answer questions")],
)

app = BedrockAgentCoreA2AApp(agent_card=agent_card)

@app.entrypoint
async def handle_message(request, context):
    user_text = request.params.get("message", {}).get("parts", [{}])[0].get("text", "")
    return {"artifacts": [{"parts": [{"kind": "text", "text": f"Response to: {user_text}"}]}]}

if __name__ == "__main__":
    app.run()

Test Plan

• All 54 unit tests pass
• Linter checks pass
• Tested locally with curl commands
• Deployed to AgentCore Runtime and verified working