Keep a reference to asyncio tasks in astream_chat() (#17812)

llama-index-core/llama_index/core/agent/legacy/react/base.py

llama-index-core/llama_index/core/agent/react/step.py

...ex-core/llama_index/core/chat_engine/condense_question.py

llama-index-core/llama_index/core/chat_engine/simple.py

llama-index-core/llama_index/core/chat_engine/types.py

llama-index-core/tests/chat_engine/test_simple.py

...ai-legacy/llama_index/agent/openai_legacy/openai_agent.py

...llama-index-agent-openai/llama_index/agent/openai/step.py