fix(pandas): Annotated[pd.DataFrame, DataframeSchema(columns=["a"])] output spec

[judahrand]

What does this PR address?

Output specs defined as Annotated[pd.DataFrame, DataframeSchema(columns=["a"])] don't work because lists are not hashable. This PR adds a converted to the columns field of DataframeSchema to convert them to a tuple.

Also adds another missing test!

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[frostming]

Thank you