Support string and timestamp MODE aggregations

[xiangfu0]

Extend the existing ModeAggregationFunction to support STRING and TIMESTAMP automatically with ordinary MODE(value) syntax. AggregationFunctionType.inferArguments supplies inferred arguments through the existing single-stage aggregate optimizer and multi-stage aggregate conversion paths. No function-specific optimizer or planner rule is needed. Complete frequency maps survive distributed stages, and timestamp milliseconds remain exact. Numeric MODE retains DOUBLE results and its existing reducers.

STRING and TIMESTAMP support MIN (default) or MAX ties; AVG is rejected. With null handling enabled, nulls are skipped and empty/all-null typed aggregates return null. Scope is grouped/global MODE, shared argument inference, and focused regression tests.

Validation: 79 OSS JVM tests pass: 56 MODE aggregation cases, eight single-stage inference cases, five planner cases, nine distributed query cases, and the existing explain-plan integration test. Coverage includes both physical planners, direct aggregation, grouping sets, and unchanged legacy rewrite options. Formatting, Checkstyle, and license checks pass on affected modules. The extra compiler-warning build could not complete because an unchanged compression dependency references an unavailable org.jetbrains.annotations.NotNull; the standard source build and tests pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.58525% with 66 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.75%. Comparing base (5e05b9c) to head (0904ef9).
⚠️ Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
...r/statement/AggregateFunctionRewriteOptimizer.java	60.25%	20 Missing and 11 partials ⚠️
...che/pinot/segment/spi/AggregationFunctionType.java	34.61%	16 Missing and 1 partial ⚠️
.../aggregation/function/ModeAggregationFunction.java	84.88%	7 Missing and 6 partials ⚠️
...apache/pinot/calcite/rel/rules/PinotRuleUtils.java	75.00%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #19489       +/-   ##
=============================================
+ Coverage     39.44%   67.75%   +28.31%     
- Complexity     1429     1430        +1     
=============================================
  Files          3488     3489        +1     
  Lines        224424   224831      +407     
  Branches      35429    35524       +95     
=============================================
+ Hits          88528   152341    +63813     
+ Misses       127912    60435    -67477     
- Partials       7984    12055     +4071     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-25	67.75% <69.58%> (+28.31%)	⬆️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	67.75% <69.58%> (+28.31%)	⬆️
unittests	67.75% <69.58%> (+28.31%)	⬆️
unittests1	57.83% <69.58%> (?)
unittests2	39.44% <10.59%> (+<0.01%)	⬆️

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