feat: Add multi-agent example using a2a SDK

[YWoooooo]

This commit introduces a new example demonstrating a multi-agent system built with the a2a SDK. The system consists of:

1. HostAgent: The main orchestrator agent.
2. PlanAgent: An agent responsible for generating a plan.
3. SearchAgent: An agent responsible for performing searches.
4. ReportAgent: An agent responsible for compiling a report.

The HostAgent coordinates these agents to process a task. It uses the A2AClient to communicate with the other agents over HTTP.

The example includes:

• Implementation of each agent.
• A main.py script to launch all agents, each on a separate port,
  using multiprocessing.
• A test_client.py script to send a task to the HostAgent and
  receive the final report, demonstrating an end-to-end workflow.
• Unit tests for each agent, ensuring individual components function
  correctly. These tests cover success cases, edge cases, and error
  handling, with mocking employed for HostAgent's dependencies on
  other agents.

This example provides a practical demonstration of how to build and orchestrate multiple agents using the a2a SDK.

Description

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Fixes #<issue_number_goes_here> 🦕

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