Cast item lists to common schema when converting to Arrow

[mdekstrand]

If item lists in a collection have different schemas, they would fail when being converted to Arrow. This change adds a test for the bug this causes, and an explicit cast to convert data to a common schema.

Closes #1164.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.61%. Comparing base (58f7ef0) to head (5815b92).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1167   +/-   ##
=======================================
  Coverage   89.61%   89.61%           
=======================================
  Files         261      261           
  Lines       17328    17329    +1     
=======================================
+ Hits        15529    15530    +1     
  Misses       1799     1799           

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