refactor: snowparkify example_helper dataframe expr

[sfc-gh-twhite]

I'm submitting this pull request to refactor the dataframe expression in the source_df definition. The current expression is a bit hard to read and maintain, so I'd like to simplify it using the table method and list concatenation.

The refactored version avoids string concatenation, which can be error-prone. I've tested the refactored code and it produces the same result as the original code.

Thank you for considering this pull request!