fix(agents): await cancelled tasks in pre-3.11 ParallelAgent merge

`_merge_agent_run_pre_3_11()` cancelled its background tasks but did
not await them, so a sibling task could still be mid-`async for` when
the caller ran `aclose()` on the same generator and hit `RuntimeError:
aclose(): asynchronous generator is already running`. That cleanup
race could mask the original sub-agent failure.

Close #5297

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 955369097

src/google/adk/agents/parallel_agent.py

@@ -158,7 +158,10 @@ async def _merge_agent_run_pre_3_11(
   finally:
     for task in tasks:
       task.cancel()
-    await asyncio.gather(*tasks, return_exceptions=True)
+    if tasks:
+      # Await cancellation so siblings are no longer mid-iteration when the
+      # caller `aclose()`s them (else "generator is already running").
+      await asyncio.gather(*tasks, return_exceptions=True)
 
 
 @deprecated(