fix: post-process scale-0 decimals in JSON stats with regex

[DrakeLin]

What changes are proposed in this pull request?

Fixes #1431 by post-processing JSON output to strip ".0" suffix from Decimal columns with scale=0.

How was this change tested?

new unit test

[codecov]

Codecov Report

❌ Patch coverage is 91.08911% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.87%. Comparing base (70143ca) to head (986d097).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...src/engine/arrow_expression/evaluate_expression.rs	91.08%	5 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1439      +/-   ##
==========================================
+ Coverage   84.85%   84.87%   +0.01%     
==========================================
  Files         119      119              
  Lines       30961    31061     +100     
  Branches    30961    31061     +100     
==========================================
+ Hits        26271    26362      +91     
- Misses       3408     3413       +5     
- Partials     1282     1286       +4     

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