fix: make run_step return all the events produced by a step (#18082)

docs/docs/module_guides/workflow/index.md

docs/docs/understanding/workflows/observability.md

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

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

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

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

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

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

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

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

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

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

...ions/embeddings/llama-index-embeddings-gemini/tests/BUILD

...ions/embeddings/llama-index-embeddings-google/tests/BUILD

...index-integrations/llms/llama-index-llms-palm/tests/BUILD

...-integrations/readers/llama-index-readers-gcs/tests/BUILD

...ievers/llama-index-retrievers-vertexai-search/tests/BUILD

...ama-index-tools-playwright/tests/test_tools_playwright.py

...r_stores/llama-index-vector-stores-lancedb/pyproject.toml