fix(python): Address potential "batch_size" parameter collision in scan_pyarrow_dataset

[alexander-beedie]

Fixes #25316 (following a Rust-side refactor for pl.defer in #21685), and adds some new test coverage.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.57%. Comparing base (8ab1657) to head (a865c69).

Files with missing lines	Patch %	Lines
...rs/src/polars/io/pyarrow_dataset/anonymous_scan.py	75.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25654      +/-   ##
==========================================
- Coverage   79.58%   79.57%   -0.01%     
==========================================
  Files        1743     1743              
  Lines      240439   240448       +9     
  Branches     3038     3040       +2     
==========================================
- Hits       191347   191343       -4     
- Misses      48310    48322      +12     
- Partials      782      783       +1     

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