[c++] Fix memory-management issues in new domainish helpers

[johnkerl]

Issue and/or context: Found while debugging memory-management issues on #3027 for #2407 / [sc-51048].

Changes:

The new libtiledbsoma helpers used by #3027 were incorrectly pairing new and free, as immediately revealed by my running valgrind build/libtiledbsoma/test/unit_soma while debugging some issues on #3027. There are code comments saying to use new to match the releaser callbacks, but these are incorrect -- the releaser callbacks are using free.

I also took the opportunity to check the ArrowArray and ArrowSchema structs and ensure I was setting all missing attributes to 0 or nullptr, respectively.

Notes for Reviewer:

It might be simpler to make use of memset(arrow_array, 0, sizeof(ArrowArray) and memset(arrow_schema, 0, sizeof(ArrowSchema) right after the allocs, to avoid all the onesie-twosie setting of non-relevant attributes to 0 or nullptr, respectively.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.79%. Comparing base (11d5396) to head (937ee36).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3030      +/-   ##
==========================================
+ Coverage   89.64%   89.79%   +0.14%     
==========================================
  Files          40       40              
  Lines        4096     4096              
==========================================
+ Hits         3672     3678       +6     
+ Misses        424      418       -6     

Flag	Coverage Δ
python	89.79% <ø> (+0.14%)	⬆️

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Components	Coverage Δ
python_api	89.79% <ø> (+0.14%)	⬆️
libtiledbsoma	∅ <ø> (∅)

[nguyenv]

Thank you for make all these consistently malloc/free.