Convert from Moq to NSubstitute

[tillig]

Summary

• Replace Moq with NSubstitute 5.3.0
• Convert new Mock<T>().Object usages to Substitute.For<T>()
• Part of cross-repo effort to standardize on NSubstitute

Test plan

• Build succeeds locally
• All 31 tests pass locally

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.24%. Comparing base (7f2f5a2) to head (c4d5087).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #18   +/-   ##
========================================
  Coverage    98.24%   98.24%           
========================================
  Files            4        4           
  Lines          114      114           
  Branches        18       18           
========================================
  Hits           112      112           
  Misses           1        1           
  Partials         1        1           

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