fix: Reading CSV with low_memory gave no data

[nameexhaustion]

Fixes #16010

I've left the code in but removed calls to them for now, as I think I will be looking to do proper refactoring later.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.83%. Comparing base (79a2cf1) to head (459bc06).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16231      +/-   ##
==========================================
- Coverage   80.92%   80.83%   -0.10%     
==========================================
  Files        1393     1394       +1     
  Lines      179568   179948     +380     
  Branches     2909     2909              
==========================================
+ Hits       145321   145454     +133     
- Misses      33742    33989     +247     
  Partials      505      505              

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[nameexhaustion]

I have adjusted this as the mmap batched reader isn't strictly respecting the batch size

pl.select(a=pl.repeat(1, 100)).write_csv(".env/data.csv")
f = pl.read_csv_batched(".env/data.csv", batch_size=5).next_batches
lst = []
while b := f(5):
    lst.extend(b)
print([x.height for x in lst])
# [11, 11, 11, 11, 11, 11, 11, 11, 11, 1]