Support v2 gapfill server-side execution

[xiangfu0]

Summary

Add server‑side gapfill support for the v2 engine, including query‑option plumbing, server response handling, and multi‑stage plan/runtime integration.

Key Changes

• Introduce serverSideGapfill and serverSideGapfillQuery query options, plus serialization/deserialization of the original PinotQuery for gapfill evaluation.

• Broker handlers (single‑stage and multi‑stage) detect eligible gapfill queries and inject server‑side gapfill options with routing/partition checks.

• Server‑side execution uses new GapfillInstanceResponseOperator and GapfillInstanceResponsePlanNode; v2 plan maker wires them in.

• Runtime/leaf planning strips GAPFILL where needed and applies gapfill in TransformOperator for MSE.

• Gapfill reducers and DateTimeConvert updated to align server‑side gapfill behavior; add GapfillIntegrationTest coverage.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 10.26316% with 682 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.02%. Comparing base (c899956) to head (6accd39).

Files with missing lines	Patch %	Lines
...sthandler/BaseSingleStageBrokerRequestHandler.java	2.53%	152 Missing and 2 partials ⚠️
.../pinot/query/runtime/operator/GapfillOperator.java	0.00%	148 Missing ⚠️
...requesthandler/MultiStageBrokerRequestHandler.java	0.00%	141 Missing ⚠️
...ache/pinot/core/query/reduce/GapfillProcessor.java	23.23%	61 Missing and 15 partials ⚠️
.../pinot/core/query/reduce/BaseGapfillProcessor.java	7.14%	37 Missing and 2 partials ⚠️
...ry/runtime/plan/server/ServerPlanRequestUtils.java	23.91%	28 Missing and 7 partials ⚠️
...he/pinot/query/runtime/plan/PlanNodeToOpChain.java	26.82%	25 Missing and 5 partials ⚠️
.../runtime/plan/server/ServerPlanRequestVisitor.java	28.94%	21 Missing and 6 partials ⚠️
...e/pinot/core/query/reduce/BrokerReduceService.java	16.66%	12 Missing and 3 partials ⚠️
...not/core/query/reduce/GapfillProcessorFactory.java	0.00%	8 Missing ⚠️
... and 4 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17533      +/-   ##
============================================
- Coverage     63.21%   63.02%   -0.19%     
+ Complexity     1476     1471       -5     
============================================
  Files          3170     3171       +1     
  Lines        189508   190249     +741     
  Branches      28997    29209     +212     
============================================
+ Hits         119789   119900     +111     
- Misses        60417    61009     +592     
- Partials       9302     9340      +38     

Flag	Coverage Δ
custom-integration1	?
integration	?
integration1	?
integration2	?
java-11	62.98% <10.26%> (-0.21%)	⬇️
java-21	62.96% <10.26%> (+7.44%)	⬆️
temurin	63.02% <10.26%> (-0.19%)	⬇️
unittests	63.02% <10.26%> (-0.19%)	⬇️
unittests1	55.45% <16.05%> (-0.11%)	⬇️
unittests2	33.91% <0.92%> (-0.12%)	⬇️

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[Jackie-Jiang]

Can you add a section in the description about the motivation of this PR?
Do we need MSE support for GAP_FILL? Should it be modeled as window function?