schema_overrides in pl.read_csv_batched requires full schema instead of partial overrides

[aruasouza]

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
import numpy as np

df = pl.DataFrame({'a':np.random.rand(100),'b':np.random.rand(100)})
df.write_csv('data.csv')

reader = pl.read_csv_batched('data.csv',schema_overrides={'a':pl.String})
batches = reader.next_batches(1)

Log output

---------------------------------------------------------------------------
ComputeError                              Traceback (most recent call last)
Cell In[31], line 5
      2 df.write_csv('data.csv')
      4 reader = pl.read_csv_batched('data.csv',schema_overrides={'a':pl.String})
----> 5 batches = reader.next_batches(1)

File ~\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.12_qbz5n2kfra8p0\LocalCache\local-packages\Python312\site-packages\polars\io\csv\batched_reader.py:133, in BatchedCsvReader.next_batches(self, n)
    109 def next_batches(self, n: int) -> list[DataFrame] | None:
    110     """
    111     Read `n` batches from the reader.
    112 
   (...)
    131     list of DataFrames
    132     """
--> 133     if (batches := self._reader.next_batches(n)) is not None:
    134         if self.new_columns:
    135             return [
    136                 _update_columns(wrap_df(df), self.new_columns) for df in batches
    137             ]

ComputeError: found more fields than defined in 'Schema'

Consider setting 'truncate_ragged_lines=True'.

Issue description

The schema_overrides parameter in pl.read_csv_batched currently requires the complete schema definition. If only a partial schema is provided (as intended for overrides), it raises an error identical to the one raised by pl.read_csv when its schema parameter is not fully specified.

Expected behavior

According to the documentation, schema_overrides should accept any number of columns, allowing users to specify only the columns they wish to override. This matches the behavior of schema_overrides in pl.read_csv, which does not require the entire schema to function.

Installed versions

--------Version info---------
Polars:              1.12.0
Index type:          UInt32
Platform:            Windows-10-10.0.19045-SP0
Python:              3.12.7 (tags/v3.12.7:0b05ead, Oct  1 2024, 03:06:41) [MSC v.1941 64 bit (AMD64)]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               
<not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               <not installed>
gevent               <not installed>
great_tables         <not installed>
matplotlib           3.9.2
nest_asyncio         1.6.0
numpy                1.26.4
openpyxl             <not installed>
pandas               2.2.3
pyarrow              17.0.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>