Add StopEvent step validation

[adrianlyjak]

Description

I was helping a colleague with a workflow. It was acting very strange, the workflow step would run up until the first call to an await, and then it would stop. No exception, just wouldn't continue. Then I realized that their workflow @step was accepting a StopEvent as the event, causing only the synchronous parts of their step to run (and the workflow to otherwise complete)

This doesn't seem like something one should ever do in a step, so this adds validation to prevent accepting StopEvent within a step handler.

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran uv run make format; uv run make lint to appease the lint gods

[logan-markewich]

The failing test in core is just flakey (need to figure out why though someday)

[adrianlyjak]

The failing test in core is just flakey (need to figure out why though someday)

That's what it looked like. I also noticed locally that there was a weird workflow test: test_deprecated_send_event, that failed the first time I made an edit to the code locally, but then would consistently pass after that 🧐 . No idea what could be causing that.