Fix issue with splitting backordered units

[adammathys]

Summary

When splitting backordered inventory units, we were incorrectly sending all backordered units to the desired shipment, regardless of the quantity chosen.

Our original work on this changed from computing on_hand units first to the backordered ones. Unfortunately, we missed that we should only ever move as many as was requested, not all of the possible backordered ones.

Related to #6319

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.35%. Comparing base (72f1b5c) to head (d368c57).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6320   +/-   ##
=======================================
  Coverage   89.35%   89.35%           
=======================================
  Files         961      961           
  Lines       20192    20194    +2     
=======================================
+ Hits        18043    18045    +2     
  Misses       2149     2149           

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[tvdeyen]

Thanks for fixing

[jarednorman]

Look great!