[Copilot]

Two tests fail against polars==1.8.0 (the declared minimum version) due to API differences.

Changes

• test_join_between_vec_as_list_expr: pl.col("list_col") + scalar raises InvalidOperationError in polars 1.8.0 — scalar arithmetic doesn't broadcast into list elements until a later release. Replaced with the explicit list.eval(element() + 1) form, which is equivalent and works across all supported versions.

• test_join_between_null_vec_row_as_counts / frame.py: In polars 1.8.0, list.len() returns 0 for null lists instead of null. The as_counts=True path was applying .list.len().cast(UInt32) inline in the expression, silently dropping nulls. Moved the conversion out of the plugin expression chain and applied it post-hoc with an explicit when/then/otherwise null guard:

clipped_series = (
    clipped_series
    .to_frame()
    .select(
        pl.when(pl.col(val_col_name).is_null())
        .then(pl.lit(None).cast(pl.UInt32))
        .otherwise(pl.col(val_col_name).list.len().cast(pl.UInt32))
        .alias(val_col_name)
    )
    [val_col_name]
)