Python: Add A2A server sample (#4528)

* Python: Add A2A server sample and fix client streaming bug

Add a pure Python A2A server sample so testing the A2A client no longer
requires running the .NET server. The server uses the a2a-sdk's
A2AStarletteApplication with uvicorn and supports three agent types
(invoice, policy, logistics) backed by AzureOpenAIResponsesClient.

New files:
- a2a_server.py: Main server entry point with CLI args
- agent_executor.py: Bridges a2a-sdk AgentExecutor to Agent Framework
- agent_definitions.py: Agent and AgentCard factory definitions
- invoice_data.py: Mock invoice data and query tool functions
- a2a_server.http: REST Client requests for testing

Also fixes a streaming bug in agent_with_a2a.py where async with was
used on ResponseStream which does not support the async context manager
protocol. Changed to async for to match all other samples.

Closes #4045

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review: handle CancelledError and fix end_date filtering

- Re-raise asyncio.CancelledError before the broad exception handler
  so cooperative cancellation is not swallowed.
- Make end_date filter inclusive of the full day by comparing with
  < end + timedelta(days=1) instead of <= midnight.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/samples/04-hosting/a2a/agent_with_a2a.py

@@ -78,16 +78,16 @@ async def main():
         #    Updates arrive as Server-Sent Events, letting you observe
         #    progress in real time as the remote agent works.
         print("\n--- Streaming response ---")
-        async with agent.run("Tell me about yourself", stream=True) as stream:
-            async for update in stream:
-                for content in update.contents:
-                    if content.text:
-                        print(f"  {content.text}")
+        stream = agent.run("Tell me about yourself", stream=True)
+        async for update in stream:
+            for content in update.contents:
+                if content.text:
+                    print(f"  {content.text}")
 
-            response = await stream.get_final_response()
-            print(f"\nFinal response ({len(response.messages)} message(s)):")
-            for message in response.messages:
-                print(f"  {message.text}")
+        response = await stream.get_final_response()
+        print(f"\nFinal response ({len(response.messages)} message(s)):")
+        for message in response.messages:
+            print(f"  {message.text}")
 
 
 if __name__ == "__main__":