SchemaFieldNotFoundError with custom check function if no alias is provided.

[philiporlando]

Describe the bug

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandera.
• (optional) I have confirmed this bug exists on the main branch of pandera.

Code Sample, a copy-pastable example

I'm trying to validate a polars dataframe using a custom check function.

import polars as pl
import pandera.polars as pa


# Custom check function
def check_custom_condition(df: pa.PolarsData) -> pl.DataFrame:
    return df.lazyframe.select(
        pl.when(
            pl.col("column1").is_null()
            & pl.col(df.key).is_null()
        )
        .then(False)
        .otherwise(True)
        # .alias("check_result")  # Uncomment this line to avoid the issue
    )

# Define the schema for the DataFrame
schema = pa.DataFrameSchema({
    "column1": pa.Column(
        dtype=str,
        nullable=True,
    ),
    "column2": pa.Column(
        dtype=str,
        nullable=True,
        checks=[
            pa.Check(check_fn=check_custom_condition),
        ],
    ),
})

# Example DataFrame
data = {
    "column1": [None, "x", "y"],
    "column2": ["a", None, "c"]
}

df = pl.DataFrame(data)

# Validate the DataFrame using the schema and custom check
schema.validate(df, lazy=True)

The example above produces the following error:

{
    "DATA": {
        "CHECK_ERROR": [
            {
                "schema": null,
                "column": "column2",
                "check": "check_custom_condition",
                "error": "SchemaFieldNotFoundError(\"literal\")"
            }
        ]
    }
}

Expected behavior

I would expect the schema validation to run successfully here. When we uncomment the .alias("check_result") line, the schema validation runs without error. I'm trying to understand if this behavior is expected, or if this is a bug.

Desktop (please complete the following information):

• OS: Windows 10 & Ubuntu 22.04.4
• Browser: Chrome
• Version: pandera==0.19.3