test: add UnionArray hashing SQL coverage.

[VaibhaveS]

Which issue does this PR close?

• Closes Add UnionArray tests exercising hashing, group-by, distinct, and aggregates #18791.

Rationale for this change

Add regression coverage for SQL operations on UnionArray columns.

What changes are included in this PR?

Adds SQL logic tests for grouping, distinct, and aggregation on UnionArray columns.

Are these changes tested?

Yes. The focused SQL logic test passes:

cargo test --profile=ci --test sqllogictests -- union_function.slt

Are there any user-facing changes?

No.

[Jefffrey]

it would be good to ensure the input dataset can actually represent the operations being done; the current input table being used only has 3 rows, all distinct, so technically theres no guarantee any of the hashing is working properly for distinct values

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.06%. Comparing base (abc5ce7) to head (85ea34e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24199      +/-   ##
==========================================
+ Coverage   81.05%   81.06%   +0.01%     
==========================================
  Files        1106     1106              
  Lines      382398   381850     -548     
  Branches   382398   381850     -548     
==========================================
- Hits       309938   309534     -404     
+ Misses      54149    54040     -109     
+ Partials    18311    18276      -35     

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[Jefffrey]

thanks @VaibhaveS