Make SimpleCoordinator inventory_units keyword arg

[sofiabesenski4]

Summary

For future improvements, we need to support additional keyword arguments to this class. The current inventory_units positional argument interferes with this.

In this commit we preserve the existing positional argument behaviour, while allowing keyword arguments to be passed in instead. This makes upgrades easier for existing projects since it does not break the existing behaviour.

Next Steps

We are working to provide a configurable replacement for the Spree::Stock::SimpleCoordinator, utilizing the middleware design pattern for increased configurability and extensibility.

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.67%. Comparing base (a0746e6) to head (b67737d).

Files with missing lines	Patch %	Lines
core/app/models/spree/stock/simple_coordinator.rb	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6483      +/-   ##
==========================================
- Coverage   89.68%   89.67%   -0.01%     
==========================================
  Files         991      991              
  Lines       20832    20835       +3     
==========================================
+ Hits        18683    18684       +1     
- Misses       2149     2151       +2     

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[jarednorman]

Just to call this out for anyone else on core reviewing it: we realized that the unit tests for the coordinator don't actually ever use this argument, which is not ideal.