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/04-hosting/a2a/a2a_server.py

@@ -9,7 +9,7 @@ from a2a.server.request_handlers import DefaultRequestHandler
 from a2a.server.routes import create_agent_card_routes, create_jsonrpc_routes
 from a2a.server.tasks import InMemoryTaskStore
 from agent_definitions import AGENT_CARD_FACTORIES, AGENT_FACTORIES
-from agent_executor import AgentFrameworkExecutor
+from agent_framework.a2a import A2AExecutor
 from agent_framework.foundry import FoundryChatClient
 from azure.identity import AzureCliCredential
 from dotenv import load_dotenv
@@ -92,7 +92,7 @@ def main() -> None:
     # Build the A2A server components
     url = f"http://{args.host}:{args.port}/"
     agent_card = AGENT_CARD_FACTORIES[args.agent_type](url)
-    executor = AgentFrameworkExecutor(agent)
+    executor = A2AExecutor(agent, stream=True)
     task_store = InMemoryTaskStore()
     request_handler = DefaultRequestHandler(
         agent_executor=executor,