Add an SSE group-key generator provider

[xiangfu0]

Summary

• add a narrow GroupKeyGeneratorProvider extension point for single-stage group-by planning
• expose immutable physical group-key metadata through GroupKeyGeneratorContext
• preserve the existing generator-selection hot path when the default provider is used
• make custom generator ownership explicit across execution, materialization, combine, and failure paths

This is an extension seam only. The default provider is an identity sentinel, so existing queries keep the current built-in group-key generators and behavior.

Extension example

class CustomPlanMaker extends InstancePlanMakerImplV2 {
  @Override
  protected GroupKeyGeneratorProvider validateAndResolveGroupKeyGeneratorProvider(
      SegmentContext segmentContext, QueryContext queryContext) {
    return context -> {
      if (context.getGroupKeySpecs().size() != 1) {
        return Optional.empty();
      }
      return createSpecializedGenerator(context);
    };
  }
}

Providers return Optional.empty() for unsupported physical shapes. Every non-empty result must be a fresh, query-owned generator and participates in Pinot's close/ownership-transfer contract. A provider reused across plans or queries must be thread-safe.

The plan-maker hook is also called for filtered group-by queries so an override can validate policy. Filtered execution continues to use Pinot's shared built-in generator and ignores the returned provider.

Validation

• 28 focused pinot-core group-by/provider/lifecycle tests
• real-segment parity for RAW, RAW with a side dictionary, dictionary fallback, and null-handling fallback
• owned-versus-borrowed StarTree initialization failure and suppressed-close coverage
• formatting, Checkstyle, and license checks for pinot-core

Scope

The provider is currently LimitedPrivate("StarTree") and Unstable. This PR does not add an off-heap implementation or a new query/config option; those live in the dependent StarTree change.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.53414% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.58%. Comparing base (bbbed25) to head (148993d).

Files with missing lines	Patch %	Lines
.../core/operator/combine/GroupByCombineOperator.java	57.50%	16 Missing and 1 partial ⚠️
...ry/aggregation/groupby/DefaultGroupByExecutor.java	82.97%	9 Missing and 7 partials ⚠️
...che/pinot/core/operator/query/GroupByOperator.java	80.00%	3 Missing and 1 partial ⚠️
...t/core/operator/query/FilteredGroupByOperator.java	85.71%	1 Missing and 2 partials ⚠️
...ore/startree/executor/StarTreeGroupByExecutor.java	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #19385      +/-   ##
============================================
+ Coverage     57.71%   67.58%   +9.87%     
- Complexity        7     1430    +1423     
============================================
  Files          2686     3488     +802     
  Lines        163987   224360   +60373     
  Branches      26627    35408    +8781     
============================================
+ Hits          94640   151637   +56997     
+ Misses        61352    60695     -657     
- Partials       7995    12028    +4033     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-25	67.58% <83.53%> (+9.87%)	⬆️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	67.58% <83.53%> (+9.87%)	⬆️
unittests	67.58% <83.53%> (+9.87%)	⬆️
unittests1	57.71% <83.53%> (+<0.01%)	⬆️
unittests2	39.29% <0.00%> (?)

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