Validate order number uniqueness only when it changes

[ikraamg]

Solidus validates number uniqueness on every order save. The validation passes case_sensitive: true, which opts out of Rails' built-in skip for unchanged values, so the uniqueness query runs on every save, including each order recalculation.

This gates the uniqueness validation on number_changed? so it only runs on create and when the number changes.

• order recalculation drops from 4 queries to 2
• uniqueness is still enforced for new orders and for number changes
• adds specs covering both the enforcement and the skip

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.02%. Comparing base (7b576fe) to head (a84c051).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6493   +/-   ##
=======================================
  Coverage   92.02%   92.02%           
=======================================
  Files        1035     1035           
  Lines       21129    21130    +1     
=======================================
+ Hits        19444    19445    +1     
  Misses       1685     1685           

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[tvdeyen]

Nice. Thank you