Add tests for parsing and sparse edge cases

[cjdoris]

Summary

• fix DATE attribute parsing to use the default ARFF date format when none is specified
• ensure parse_javadateformat reports the original format when quotes are unbalanced
• add comprehensive parsing and loading test items covering quote heuristics, sparse zero fill, and unsupported column kinds
• add save test verifying AbstractString conversions

Testing

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

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (bb111e3) to head (f7cf2bb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   81.92%   90.25%   +8.33%     
==========================================
  Files           1        1              
  Lines         708      708              
==========================================
+ Hits          580      639      +59     
+ Misses        128       69      -59     

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