schema_overrides should raise an error on non-existing keys

[stinodego]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl

df = pl.DataFrame({"a": [1, 2], "b": [3, 4]}, schema_overrides={"c": pl.Int8})

Log output

shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 3   │
│ 2   ┆ 4   │
└─────┴─────┘

Issue description

Non-existing keys in schema_overrides are ignored.

Expected behavior

An ValueError should be raised.

Installed versions

main

[stinodego]

Related to #11723

[stinodego]

We actually decided against this in the linked issue.