fix(agents): await cancelled tasks in _merge_agent_run_pre_3_11 to prevent aclose() RuntimeError#5416

Koushik-Salammagari wants to merge 3 commits intogoogle:maingoogle/adk-python:mainfrom

Koushik-Salammagari:fix/parallel-agent-pre-311-aclose-errorKoushik-Salammagari/adk-python:fix/parallel-agent-pre-311-aclose-errorCopy head branch name to clipboard