Python: basic python a2a support (#906)

* basic python a2a support

* fixes

* small fixes

---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

python/samples/getting_started/agents/a2a/README.md

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+# A2A Agent Examples
+
+This folder contains examples demonstrating how to create and use agents with the A2A (Agent-to-Agent) protocol from the `agent_framework` package to communicate with remote A2A agents.
+
+For more information about the A2A protocol specification, visit: https://a2a-protocol.org/latest/
+## Examples
+
+| File | Description |
+|------|-------------|
+| [`agent_with_a2a.py`](agent_with_a2a.py) | The simplest way to connect to and use a single A2A agent. Demonstrates agent discovery via agent cards and basic message exchange using the A2A protocol. |
+
+## Environment Variables
+
+Make sure to set the following environment variables before running the example:
+
+### Required
+- `A2A_AGENT_HOST`: URL of a single A2A agent (for simple sample, e.g., `http://localhost:5001/`)
+
+
+## Quick Testing with .NET A2A Servers
+
+For quick testing and demonstration, you can use the pre-built .NET A2A servers from this repository:
+
+**Quick Testing Reference**: Use the .NET A2A Client Server sample at:
+`..\agent-framework\dotnet\samples\A2AClientServer`
+
+### Run Python A2A Sample
+```powershell
+# Simple A2A sample (single agent)
+uv run python agent_with_a2a.py
+```

python/samples/getting_started/agents/a2a/agent_with_a2a.py

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+# Copyright (c) Microsoft. All rights reserved.
+
+import asyncio
+import os
+
+import httpx
+from a2a.client import A2ACardResolver
+from agent_framework.a2a import A2AAgent
+
+"""
+Agent-to-Agent (A2A) Protocol Integration Sample
+
+This sample demonstrates how to connect to and communicate with external agents using
+the A2A protocol. A2A is a standardized communication protocol that enables interoperability
+between different agent systems, allowing agents built with different frameworks and
+technologies to communicate seamlessly.
+
+For more information about the A2A protocol specification, visit: https://a2a-protocol.org/latest/
+
+Key concepts demonstrated:
+- Discovering A2A-compliant agents using AgentCard resolution
+- Creating A2AAgent instances to wrap external A2A endpoints
+- Converting Agent Framework messages to A2A protocol format
+- Handling A2A responses (Messages and Tasks) back to framework types
+
+To run this sample:
+1. Set the A2A_AGENT_HOST environment variable to point to an A2A-compliant agent endpoint
+   Example: export A2A_AGENT_HOST="https://your-a2a-agent.example.com"
+2. Ensure the target agent exposes its AgentCard at /.well-known/agent.json
+3. Run: uv run python agent_with_a2a.py
+
+The sample will:
+- Connect to the specified A2A agent endpoint
+- Retrieve and parse the agent's capabilities via its AgentCard
+- Send a message using the A2A protocol
+- Display the agent's response
+
+Visit the README.md for more details on setting up and running A2A agents.
+"""
+
+
+async def main():
+    """Demonstrates connecting to and communicating with an A2A-compliant agent."""
+    # Get A2A agent host from environment
+    a2a_agent_host = os.getenv("A2A_AGENT_HOST")
+    if not a2a_agent_host:
+        raise ValueError("A2A_AGENT_HOST environment variable is not set")
+
+    print(f"Connecting to A2A agent at: {a2a_agent_host}")
+
+    # Initialize A2ACardResolver
+    async with httpx.AsyncClient(timeout=60.0) as http_client:
+        resolver = A2ACardResolver(httpx_client=http_client, base_url=a2a_agent_host)
+
+        # Get agent card
+        agent_card = await resolver.get_agent_card(relative_card_path="/.well-known/agent.json")
+        print(f"Found agent: {agent_card.name} - {agent_card.description}")
+
+        # Create A2A agent instance
+        agent = A2AAgent(
+            name=agent_card.name, description=agent_card.description, agent_card=agent_card, url=a2a_agent_host
+        )
+
+        # Invoke the agent and output the result
+        print("\nSending message to A2A agent...")
+        response = await agent.run("Tell me a joke about a pirate.")
+
+        # Print the response
+        print("\nAgent Response:")
+        for message in response.messages:
+            print(message.text)
+
+
+if __name__ == "__main__":
+    asyncio.run(main())