[wangyinz]

Summary

• Return an empty result for empty distributed partitions.
• Convert DataFrame rows in partition order and report all conversion errors once, in that same order.
• Close internally created MongoDB cursors on success and failure paths.
• Add Dask, Spark, and cursor-lifecycle regression coverage.

Closes #835

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.10%. Comparing base (5ebd29d) to head (a80cdd0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #977      +/-   ##
==========================================
+ Coverage   73.07%   73.10%   +0.02%     
==========================================
  Files         182      182              
  Lines       36169    36173       +4     
  Branches     2532     2532              
==========================================
+ Hits        26432    26445      +13     
+ Misses       8832     8823       -9     
  Partials      905      905              

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[wangyinz]

Review decision: ready after final CI. Empty partitions return an empty result; DataFrame conversion reports every row error once and in input order; internally created cursors close on all paths. The combined #976/#977 suite passes (24 passed, 4 Mongo-dependent tests skipped locally; full CI supplies MongoDB).