test: xfail legacy flaky test

`tests/integration/test_scenarios.py::test_scenario_initial_task_types[new_task-streaming-legacy]` is flaky, given that it's legacy and is not used - `xfail` it.

Example failures:

- https://github.com/a2aproject/a2a-python/actions/runs/24995840443/job/73192514384
- https://github.com/a2aproject/a2a-python/actions/runs/25041293011/job/73344930927

Author: ishymko

tests/integration/test_scenarios.py

@@ -2,7 +2,6 @@
 import collections
 import contextlib
 import logging
-
 from typing import Any
 
 import grpc
@@ -13,6 +12,7 @@
 from a2a.client.client import ClientConfig
 from a2a.client.client_factory import ClientFactory
 from a2a.client.errors import A2AClientError
+from a2a.helpers.proto_helpers import new_task_from_user_message
 from a2a.server.agent_execution import AgentExecutor, RequestContext
 from a2a.server.context import ServerCallContext
 from a2a.server.events import EventQueue
@@ -47,16 +47,14 @@
     TaskStatus,
     TaskStatusUpdateEvent,
 )
-from a2a.helpers.proto_helpers import new_task_from_user_message
 from a2a.utils import TransportProtocol
 from a2a.utils.errors import (
+    InvalidAgentResponseError,
     InvalidParamsError,
     TaskNotCancelableError,
     TaskNotFoundError,
-    InvalidAgentResponseError,
 )
 
-
 logger = logging.getLogger(__name__)
 
 
@@ -1933,8 +1931,21 @@ async def listen_to_end():
 )
 @pytest.mark.parametrize('initial_task_type', ['new_task', 'status_update'])
 async def test_scenario_initial_task_types(
-    use_legacy, streaming, initial_task_type
+    request, use_legacy, streaming, initial_task_type
 ):
+    if use_legacy and streaming and initial_task_type == 'new_task':
+        # There is a race condition which manifests itself in flaky CI failures.
+        # Given that we don't use legacy by default anymore, xfail it.
+        # Flakiness rate is around 1 failure per 30 runs, so use strict=False
+        # to avoid failing on a passing xfail.
+        request.node.add_marker(
+            pytest.mark.xfail(
+                reason='https://github.com/a2aproject/a2a-python/issues/869',
+                strict=False,
+                raises=ValueError,
+            )
+        )
+
     started_event = asyncio.Event()
     continue_event = asyncio.Event()