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

[Koushik-Salammagari]

Link to Issue or Description of Change

Fixes #5297

Description

On Python 3.10, ParallelAgent uses _merge_agent_run_pre_3_11 instead of
asyncio.TaskGroup. When a sub-agent raises an exception, the finally block
cancelled all internal tasks with task.cancel() but did not await them.
This left the 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

This secondary error masked the original sub-agent exception (e.g., a
Pydantic validation failure from a structured-output agent).

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

Changes

• src/google/adk/agents/parallel_agent.py: one line added to _merge_agent_run_pre_3_11 finally block
• tests/unittests/agents/test_parallel_agent.py: regression test that directly exercises _merge_agent_run_pre_3_11 with a slow generator + failing generator, then calls aclose() — without the fix this raises RuntimeError

Testing Plan

• New test test_merge_agent_run_pre_3_11_no_aclose_error_on_failure added to tests/unittests/agents/test_parallel_agent.py
• All 9 existing parallel agent tests continue to pass

pytest tests/unittests/agents/test_parallel_agent.py -v
======================== 9 passed in 5.46s ===========================