Python: Add long-running agents and background responses support (#3808)

* Python: Add long-running agents and background responses support

- Add ContinuationToken TypedDict to core types
- Add continuation_token field to ChatResponse, ChatResponseUpdate,
  AgentResponse, and AgentResponseUpdate
- Add background and continuation_token options to OpenAIResponsesOptions
- Implement polling via responses.retrieve() and streaming resumption
  in RawOpenAIResponsesClient
- Propagate continuation tokens through agent run() and
  map_chat_to_agent_update
- Fix streaming telemetry 'Failed to detach context' error in both
  ChatTelemetryLayer and AgentTelemetryLayer by avoiding
  trace.use_span() context attachment for async-managed spans
- Add 14 unit tests for continuation token types and background flows
- Add background_responses sample showing polling and stream resumption

Fixes #2478

* Python: Add A2A long-running task support via ContinuationToken

- Make ContinuationToken provider-agnostic (total=False, optional task_id/context_id fields)
- Add background param to A2AAgent.run() controlling token emission
- Add poll_task() for single-request task state retrieval
- Add resubscribe support via continuation_token param on run()
- Extract _updates_from_task() and _map_a2a_stream() for cleaner code
- Streamline run()/streaming by removing intermediate _stream_updates wrapper
- Update A2A sample to show background=False (default) with link to background_responses sample
- Remove stale BareAgent from __all__
- Add 12 new A2A continuation token tests

* fix logic for overriding continuation token when done

* refactored ContinuationToken setup

python/samples/concepts/background_responses.py

@@ -0,0 +1,139 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+import asyncio
+
+from agent_framework import ChatAgent
+from agent_framework.openai import OpenAIResponsesClient
+
+"""Background Responses Sample.
+
+This sample demonstrates long-running agent operations using the OpenAI
+Responses API ``background`` option.  Two patterns are shown:
+
+1. **Non-streaming polling** – start a background run, then poll with the
+   ``continuation_token`` until the operation completes.
+2. **Streaming with resumption** – start a background streaming run, simulate
+   an interruption, and resume from the last ``continuation_token``.
+
+Prerequisites:
+  - Set the ``OPENAI_API_KEY`` environment variable.
+  - A model that benefits from background execution (e.g. ``o3``).
+"""
+
+
+# 1. Create the agent with an OpenAI Responses client.
+agent = ChatAgent(
+    name="researcher",
+    instructions="You are a helpful research assistant. Be concise.",
+    chat_client=OpenAIResponsesClient(model_id="o3"),
+)
+
+
+async def non_streaming_polling() -> None:
+    """Demonstrate non-streaming background run with polling."""
+    print("=== Non-Streaming Polling ===\n")
+
+    thread = agent.get_new_thread()
+
+    # 2. Start a background run — returns immediately.
+    response = await agent.run(
+        messages="Briefly explain the theory of relativity in two sentences.",
+        thread=thread,
+        options={"background": True},
+    )
+
+    print(f"Initial status: continuation_token={'set' if response.continuation_token else 'None'}")
+
+    # 3. Poll until the operation completes.
+    poll_count = 0
+    while response.continuation_token is not None:
+        poll_count += 1
+        await asyncio.sleep(2)
+        response = await agent.run(
+            thread=thread,
+            options={"continuation_token": response.continuation_token},
+        )
+        print(f"  Poll {poll_count}: continuation_token={'set' if response.continuation_token else 'None'}")
+
+    # 4. Done — print the final result.
+    print(f"\nResult ({poll_count} poll(s)):\n{response.text}\n")
+
+
+async def streaming_with_resumption() -> None:
+    """Demonstrate streaming background run with simulated interruption and resumption."""
+    print("=== Streaming with Resumption ===\n")
+
+    thread = agent.get_new_thread()
+
+    # 2. Start a streaming background run.
+    last_token = None
+    stream = agent.run(
+        messages="Briefly list three benefits of exercise.",
+        stream=True,
+        thread=thread,
+        options={"background": True},
+    )
+
+    # 3. Read some chunks, then simulate an interruption.
+    chunk_count = 0
+    print("First stream (before interruption):")
+    async for update in stream:
+        last_token = update.continuation_token
+        if update.text:
+            print(update.text, end="", flush=True)
+        chunk_count += 1
+        if chunk_count >= 3:
+            print("\n  [simulated interruption]")
+            break
+
+    # 4. Resume from the last continuation token.
+    if last_token is not None:
+        print("Resumed stream:")
+        stream = agent.run(
+            stream=True,
+            thread=thread,
+            options={"continuation_token": last_token},
+        )
+        async for update in stream:
+            if update.text:
+                print(update.text, end="", flush=True)
+
+    print("\n")
+
+
+async def main() -> None:
+    await non_streaming_polling()
+    await streaming_with_resumption()
+
+
+if __name__ == "__main__":
+    asyncio.run(main())
+
+"""
+Sample output:
+
+=== Non-Streaming Polling ===
+
+Initial status: continuation_token=set
+  Poll 1: continuation_token=set
+  Poll 2: continuation_token=None
+
+Result (2 poll(s)):
+The theory of relativity, developed by Albert Einstein, consists of special
+relativity (1905), which shows that the laws of physics are the same for all
+non-accelerating observers and that the speed of light is constant, and general
+relativity (1915), which describes gravity as the curvature of spacetime caused
+by mass and energy.
+
+=== Streaming with Resumption ===
+
+First stream (before interruption):
+Here are three
+  [simulated interruption]
+Resumed stream:
+key benefits of regular exercise:
+
+1. **Improved cardiovascular health** ...
+2. **Better mental health** ...
+3. **Stronger muscles and bones** ...
+"""