refactor(rust): Make GetBatchState polling functions unsafe

[nameexhaustion]

Hides the actual impls as private functions and marks them unsafe. The struct now exposes an async next() instead that can be safely used.

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.37%. Comparing base (ffb34bf) to head (c7299e0).

Files with missing lines	Patch %	Lines
crates/polars-stream/src/nodes/io_sources/batch.rs	84.21%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23795      +/-   ##
==========================================
- Coverage   81.39%   81.37%   -0.03%     
==========================================
  Files        1652     1652              
  Lines      224344   224356      +12     
  Branches     2872     2872              
==========================================
- Hits       182609   182566      -43     
- Misses      41016    41071      +55     
  Partials      719      719              

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