fix: Decompress moved out of schema initialization

[leoforney]

#14192

read_csv wouldn't decompress file unless schema had None value.

Ran with test file :

import polars as pl
import gzip
import os

df = pl.DataFrame({
    "id": [1, 2, 3, 4, 5],
    "name": ["Alice", "Bob", "Charlie", "David", "Eve"],
    "age": [25, 32, 37, 45, 29],
    "city": ["New York", "Los Angeles", "Chicago", "Houston", "Phoenix"]
})

temp_filename = 'tempfile.csv'
df.write_csv(temp_filename)

compressed_filename = 'dataframe.csv.gz'
with open(temp_filename, 'rb') as temp_file:
    with gzip.open(compressed_filename, 'wb') as gzip_file:
        gzip_file.writelines(temp_file)

os.remove(temp_filename)

print("DataFrame is successfully exported to a gzipped CSV file.")

schema = {
    "id": pl.Int64,
    "name": pl.Utf8,
    "age": pl.Int64,
    "city": pl.Utf8,
}

df_reopened = pl.read_csv(
        'dataframe.csv.gz',
        schema=schema,
        has_header=True
    )

print(df_reopened)

[codspeed-hq]

CodSpeed Performance Report

Merging #15550 will not alter performance

_{Comparing leoforney:main (99029d7) with main (b91dedb)}

Summary

✅ 22 untouched benchmarks

[ritchie46]

Thanks, can you add a test on the python side as well?

[leoforney]

Added!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.16%. Comparing base (44f1097) to head (99029d7).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15550      +/-   ##
==========================================
- Coverage   81.16%   81.16%   -0.01%     
==========================================
  Files        1367     1367              
  Lines      175307   175318      +11     
  Branches     2527     2530       +3     
==========================================
+ Hits       142296   142303       +7     
- Misses      32534    32541       +7     
+ Partials      477      474       -3     

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[ritchie46]

Thanks!