Handle Object-returning scalar transform functions

[wolfkill]

Summary

• handle scalar transform functions whose Java return type is Object without exposing ColumnDataType.OBJECT through transform result metadata
• fetch values for scalar function parameters declared as Object from child transform functions
• add coverage for nullIf with an Object result and an Object parameter

Fixes #11743

Tests

• JAVA_HOME=/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home ./mvnw -pl pinot-core -am -Dtest=ScalarTransformFunctionWrapperTest#testObjectResultScalarTransformFunction+testObjectParameterScalarTransformFunction -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false test
• JAVA_HOME=/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home ./mvnw -pl pinot-core -am -Dtest=ScalarTransformFunctionWrapperTest -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false test
• git diff --check

[codecov-commenter]

Codecov Report

❌ Patch coverage is 21.05263% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.38%. Comparing base (a762254) to head (693e431).

Files with missing lines	Patch %	Lines
...sform/function/ScalarTransformFunctionWrapper.java	21.05%	28 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18628      +/-   ##
============================================
- Coverage     64.40%   64.38%   -0.02%     
  Complexity     1137     1137              
============================================
  Files          3337     3337              
  Lines        206069   206106      +37     
  Branches      32128    32135       +7     
============================================
- Hits         132710   132707       -3     
- Misses        62726    62766      +40     
  Partials      10633    10633              

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-21	64.38% <21.05%> (-0.02%)	⬇️
temurin	64.38% <21.05%> (-0.02%)	⬇️
unittests	64.38% <21.05%> (-0.02%)	⬇️
unittests1	56.80% <21.05%> (-0.02%)	⬇️
unittests2	36.92% <0.00%> (-0.01%)	⬇️

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[xiangfu0]

Found a high-signal issue; see inline comment.

[xiangfu0]

This changes every OBJECT-returning scalar UDF to advertise STRING output and stringify the value. That silently changes query semantics for functions like nullIf/caseWhen: NULLIF(int_col, 0) now becomes a VARCHAR result instead of preserving the input type, so downstream arithmetic/comparisons will see the wrong type. We need a type-preserving resolution here rather than coercing OBJECT to STRING.