feat(a2a): Add A2AClient with synchronous and streaming API

[minorun365]

Summary

Add A2AClient class for communicating with remote A2A agents deployed on AWS Bedrock AgentCore Runtime.

This PR provides an initial implementation with synchronous and streaming APIs. Async API will be added in a follow-up PR to keep the PR size manageable.

Features

• A2AClient.from_agentcore_arn() for AgentCore Runtime connection with IAM auth
• A2AClient.from_url() for local development without auth
• send_task() synchronous API for sending tasks
• send_task_streaming() streaming API for real-time responses
• get_agent_card() for retrieving agent metadata
• Utility functions: build_agentcore_url, extract_region_from_arn

Planned for follow-up PR

• Async API (send_task_async(), get_agent_card_async(), send_task_streaming_async())

Usage

from strands.multiagent.a2a import A2AClient

# Connect to AgentCore Runtime (IAM auth via mcp-proxy-for-aws)
client = A2AClient.from_agentcore_arn(
    "arn:aws:bedrock-agentcore:us-east-1:123456789012:runtime/my-agent"
)

# Synchronous API
result = client.send_task("Calculate 3 + 5")

# Streaming API
for chunk in client.send_task_streaming("Write an essay"):
    print(chunk, end="", flush=True)

# Local development (no auth)
local_client = A2AClient.from_url("http://localhost:9000")

Dependencies

• Uses mcp-proxy-for-aws for SigV4 authentication (optional, only needed for AgentCore)

Test plan

• 21 unit tests added (all passing)
• ruff linting: passed
• ruff formatting: passed