test: prove first execution succeeds after reset_state

[laverem]

Follow-up to the DataFusion 54 upgrade (#4525). Adds unit tests for
ArrowStreamBatchGenerator in python/src/writer.rs covering DF54's
LazyMemoryExec::execute behavior, which calls reset_state on the first
execution:

• first reset_state hands the reader to a fresh generator that yields the
  original batch (the regression that was breaking the first write)
• a second reset_state is exhausted and errors loudly rather than silently
  yielding no rows

AI disclosure: tests were AI-generated and human verified locally with
cargo test --lib writer::tests.

[ethan-tyler]

Thanks for the follow up!! I hate to be nit picky but the AI comments are overly verbose and the datafusion version is irrelevant.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.90%. Comparing base (6420bc1) to head (739267a).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4530      +/-   ##
==========================================
+ Coverage   79.84%   79.90%   +0.05%     
==========================================
  Files         172      172              
  Lines       62882    62915      +33     
  Branches    62882    62915      +33     
==========================================
+ Hits        50211    50274      +63     
+ Misses       9722     9691      -31     
- Partials     2949     2950       +1     

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