Add tests for sparse defaults in strict mode

[cjdoris]

Summary

• add regression test covering sparse rows without explicit values when strict missing rules are applied
• ensure categorical and string defaults are asserted and warning logging is exercised

Testing

• julia --project=@. -e 'using Pkg; Pkg.test(coverage=true)'

---

Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.73%. Comparing base (1643f3f) to head (832e18d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   90.25%   98.73%   +8.47%     
==========================================
  Files           1        1              
  Lines         708      709       +1     
==========================================
+ Hits          639      700      +61     
+ Misses         69        9      -60     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.