Exception when MedRecord is created from Polars DataFrame with pl.int8 values

[JabobKrauskopf]

Check what the underlying issue is and fix. Could be related to #284

To reproduce:

import polars as pl
import medmodels as mm

test_data = pl.DataFrame(
    {
        "ID_PAT": [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
        "SMALL_INT": [4, -6, 30, -42, -37, 60, -105, 49, -88, 36],
    }
)

test_data = test_data.with_columns([
    pl.col("ID_PAT").cast(pl.Int64),
    pl.col("SMALL_INT").cast(pl.Int8)
])


medrecord = mm.MedRecord.builder().add_nodes((test_data, "ID_PAT"), group="patients").build()