Update callCount and thrownErrors properties to use checked locks

[graycampbell]

📝 Summary

• Updated callCount and thrownErrors properties to use checked locks since Int and Error are both Sendable.

🛠️ Type of Change

• Bug fix (change that fixes an issue)
• New feature (change that adds functionality)
• Breaking change (bug fix or feature that is not backwards compatible)
• Documentation (DocC, API docs, markdown files, templates, etc.)
• Testing (new tests, updated tests, etc.)
• Refactoring or code formatting (no logic changes)
• Updating dependencies (Swift packages, Homebrew, etc.)
• CI/CD (change to automated workflows)
• Chore (other maintenance)

🧪 How Has This Been Tested?

• Existing tests all pass.

🔗 Related PRs or Issues

• Resolves [Maintenance] Use checked locks for call count and thrown errors #124.

✅ Checklist

• I have added relevant tests
• I have verified all tests pass
• I have formatted my code using SwiftFormat
• I have updated documentation (if needed)
• I have added the appropriate label to my PR
• I have read the contributing guidelines
• I agree to follow this project's Code of Conduct

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.65%. Comparing base (68c4ead) to head (e15e85b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...kPropertyAsyncGetter/MockPropertyAsyncGetter.swift	0.00%	2 Missing ⚠️
...rowingGetter/MockPropertyAsyncThrowingGetter.swift	0.00%	2 Missing ⚠️
...essors/MockPropertyGetter/MockPropertyGetter.swift	0.00%	2 Missing ⚠️
...essors/MockPropertySetter/MockPropertySetter.swift	0.00%	2 Missing ⚠️
...rtyThrowingGetter/MockPropertyThrowingGetter.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage   69.65%   69.65%           
=======================================
  Files          73       73           
  Lines        4811     4811           
=======================================
  Hits         3351     3351           
  Misses       1460     1460           

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