fix: stepwise execution breaks when steps do async work (#17914)

docs/docs/module_guides/workflow/index.md

llama-index-core/llama_index/core/workflow/context.py

llama-index-core/llama_index/core/workflow/events.py

llama-index-core/llama_index/core/workflow/handler.py

llama-index-core/llama_index/core/workflow/utils.py

llama-index-core/llama_index/core/workflow/workflow.py

llama-index-core/tests/workflow/test_stepwise.py