test: improve lpad sqllogictest coverage

[2010YOUY01]

Which issue does this PR close?

• Closes #.

Rationale for this change

Codecov reports low test coverage for this function, so this PR adds more SLT tests for it.

The tests were AI-generated, and I manually verified that the results are correct.

See Codecov bot comment -> ☔ View full report in Codecov by Harness -> Indirect Changes for the updated test coverage.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.76%. Comparing base (02a902c) to head (237ee49).
⚠️ Report is 113 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23969      +/-   ##
==========================================
+ Coverage   80.71%   80.76%   +0.05%     
==========================================
  Files        1089     1096       +7     
  Lines      368293   373282    +4989     
  Branches   368293   373282    +4989     
==========================================
+ Hits       297250   301468    +4218     
- Misses      53332    53831     +499     
- Partials    17711    17983     +272     

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