fix: Convert Polars container dtype entries to list in _get_cell (#778)

[KristijanArmeni]

Summary

This pull request adds a small fix that ensures that _get_cell returns a list rather than pl.Series if cell entry is a container dtype (pl.List or pl.Array). This makes its behavior with polars dataframes consistent with _get_cell for pandas data frames which return lists for container dtypes.

I went ahead and thought this was the best place to address #778 which surfaced as a syntax bug when running one of the gt-extras tutorials with polars. Happy to consider other solutions in case it warrants a fix elsewhere in the code base!

Changes

• _tbl_data._get_cell: check if retrieved cell entry is either a container data type and convert to list.
• tests/test_tbl_data.py: adds df_container_dtypes fixture and test_get_cell_container_dtypes test

Related GitHub Issues and PRs

• Ref: Bug: PlDataFrame._get_cell() type inconsistency breaks gt-extras gt_plt_bar_stack #778

Checklist

• I understand and agree to the Code of Conduct.
• I have followed the Style Guide for Python Code as best as possible for the submitted code.
• I have added pytest unit tests for any new functionality.