Confusing error messages for primary key dtype mismatch #52

Some methods of the DataFrameComparison yield unrelated error messages if the left and the right data frame's primary keys have mismatching dtypes:

import polars as pl
from diffly import compare_frames

left = pl.DataFrame({"a": [1, 2]})
right = pl.DataFrame({"a": [1.0, 2.0]})

c = compare_frames(left, right, primary_key=["a"])
c.left_only()

raises

PrimaryKeyError: `primary_key` must be provided to join `left` and `right`.

and

c.summary()

warns

Attention: the data frames do not match exactly, but as no primary key columns are
provided, the row and column matches cannot be computed.