Python: Reorganize A2A samples and use package A2AExecutor (#6165)

* Reorganize A2A samples: client demos in 02-agents, use package A2AExecutor

- Move client samples (agent_with_a2a, a2a_agent_as_function_tools) to samples/02-agents/a2a/
- Add new concept samples: polling, stream reconnection, protocol selection
- Replace sample agent_executor.py with package-level A2AExecutor (stream=True)
- Update 04-hosting/a2a to focus on server-side, point to 02-agents for clients
- Add README.md for the new 02-agents/a2a/ sample collection

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

* Fix streaming artifact coalescing and address PR review feedback

A2AExecutor fix:
- Generate a stable artifact_id per stream in _run_stream so all streaming
  chunks share the same ID, enabling proper append=True coalescing per the
  A2A spec (TaskArtifactUpdateEvent with same artifactId).
- Previously, item.message_id was None for OpenAI/Foundry streaming updates,
  causing the SDK to generate a new random UUID per token (100+ separate
  artifacts instead of 1 appended artifact).

Sample improvements:
- Replace join workaround with response.text now that coalescing works
- Add background=True to stream reconnection resume call (required for
  continuation token emission on in-progress tasks)
- Fix type ignore specificity in polling sample

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

---------

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

python/samples/02-agents/a2a/a2a_polling.py

@@ -0,0 +1,96 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+import asyncio
+import os
+
+import httpx
+from a2a.client import A2ACardResolver
+from agent_framework.a2a import A2AAgent
+from dotenv import load_dotenv
+
+load_dotenv()
+
+"""
+A2A Polling for Task Completion
+
+This sample demonstrates how to poll a long-running A2A task for completion
+using continuation tokens. When `background=True`, the agent returns immediately
+with a continuation token that you can use to check progress later.
+
+Key concepts demonstrated:
+- Starting a background A2A task with `background=True`
+- Receiving a continuation token for in-progress tasks
+- Polling with `poll_task()` until the task reaches a terminal state
+
+This is the A2A equivalent of the .NET A2AAgent_PollingForTaskCompletion sample.
+
+Prerequisites:
+- Set A2A_AGENT_HOST to the URL of a running A2A server
+
+To run this sample:
+    cd python/samples/02-agents/a2a
+    uv run python a2a_polling.py
+"""
+
+
+async def main() -> None:
+    """Demonstrates polling a long-running A2A task for completion."""
+    a2a_agent_host = os.getenv("A2A_AGENT_HOST")
+    if not a2a_agent_host:
+        raise ValueError("A2A_AGENT_HOST environment variable is not set")
+
+    # 1. Resolve agent card and create agent.
+    async with httpx.AsyncClient(timeout=60.0) as http_client:
+        resolver = A2ACardResolver(httpx_client=http_client, base_url=a2a_agent_host)
+        agent_card = await resolver.get_agent_card()
+
+    async with A2AAgent(
+        name=agent_card.name,
+        agent_card=agent_card,
+        url=a2a_agent_host,
+    ) as agent:
+        # 2. Start a background task — the agent returns immediately.
+        print("Starting background task...")
+        response = await agent.run(
+            "Write a detailed research report on quantum computing advances in 2025",
+            background=True,
+        )
+
+        # 3. Check if we got a continuation token (task still in progress).
+        if response.continuation_token is None:
+            # Task completed immediately — no polling needed.
+            print("Task completed immediately:")
+            print(f"  {response.text}")
+            return
+
+        # 4. Poll until the task completes.
+        token = response.continuation_token
+        poll_count = 0
+        while token is not None:
+            poll_count += 1
+            print(f"  Poll #{poll_count} — task still in progress, waiting 2s...")
+            await asyncio.sleep(2)
+
+            response = await agent.poll_task(token)  # type: ignore[arg-type]
+            token = response.continuation_token
+
+        # 5. Task is done — print the final response.
+        print(f"\nTask completed after {poll_count} poll(s):")
+        print(f"  {response.text[:200]}...")
+
+
+if __name__ == "__main__":
+    asyncio.run(main())
+
+
+"""
+Sample output:
+
+Starting background task...
+  Poll #1 — task still in progress, waiting 2s...
+  Poll #2 — task still in progress, waiting 2s...
+  Poll #3 — task still in progress, waiting 2s...
+
+Task completed after 3 poll(s):
+  Quantum computing has seen remarkable progress in 2025, with breakthroughs in...
+"""