fix(agents): await cancelled tasks in _merge_agent_run_pre_3_11 to prevent aclose() RuntimeError

On Python 3.10, ParallelAgent uses _merge_agent_run_pre_3_11 instead of
asyncio.TaskGroup. When a sub-agent raises, the finally block cancelled
all tasks but did not await them. This left the internal process_an_agent
coroutines still executing their own finally blocks (which hold references
to the sub-agent async generators) when _run_async_impl subsequently called
aclose() on those generators, raising:

  RuntimeError: aclose(): asynchronous generator is already running

Fix: add asyncio.gather(*tasks, return_exceptions=True) after cancellation
so that all tasks — and their generator cleanup — complete before the caller
can invoke aclose().

Fixes #5297

Author: Koushik-Salammagari

src/google/adk/agents/parallel_agent.py

@@ -145,6 +145,7 @@ async def process_an_agent(events_for_one_agent):
   finally:
     for task in tasks:
       task.cancel()
+    await asyncio.gather(*tasks, return_exceptions=True)
 
 
 class ParallelAgent(BaseAgent):

tests/unittests/agents/test_parallel_agent.py

@@ -20,6 +20,7 @@
 from google.adk.agents.base_agent import BaseAgent
 from google.adk.agents.base_agent import BaseAgentState
 from google.adk.agents.invocation_context import InvocationContext
+from google.adk.agents.parallel_agent import _merge_agent_run_pre_3_11
 from google.adk.agents.parallel_agent import ParallelAgent
 from google.adk.agents.sequential_agent import SequentialAgent
 from google.adk.agents.sequential_agent import SequentialAgentState
@@ -373,3 +374,38 @@ async def test_stop_agent_if_sub_agent_fails(
     async for _ in agen:
       # The infinite agent could iterate a few times depending on scheduling.
       pass
+
+
+async def _slow_agent_with_cleanup_delay():
+  """Async generator that sleeps in its finally block to simulate cleanup."""
+  try:
+    await asyncio.sleep(10)
+    yield 'slow-event'
+  finally:
+    await asyncio.sleep(0.05)
+
+
+async def _failing_agent():
+  """Async generator that raises after a short delay."""
+  await asyncio.sleep(0.01)
+  raise ValueError('simulated sub-agent failure')
+  yield  # pragma: no cover
+
+
+@pytest.mark.asyncio
+async def test_merge_agent_run_pre_3_11_no_aclose_error_on_failure():
+  """Regression test for Python 3.10 RuntimeError: aclose() already running.
+
+  _merge_agent_run_pre_3_11 must await all cancelled tasks before returning so
+  that generators are fully released before the caller invokes aclose() on them.
+  """
+  agent_runs = [_slow_agent_with_cleanup_delay(), _failing_agent()]
+
+  with pytest.raises(ValueError, match='simulated sub-agent failure'):
+    async for _ in _merge_agent_run_pre_3_11(agent_runs):
+      pass
+
+  # If tasks were not properly awaited, aclose() on a still-running generator
+  # would raise RuntimeError here.
+  for agen in agent_runs:
+    await agen.aclose()