Add broker config to enable streaming group-by by default for a cluster

[yashmayya]

Summary

Follow-up to #18035. Adds a broker-level config to opt a cluster into the MSE streaming group-by leaf-stage operator by default, while leaving per-query control to operators and users.

• New broker config pinot.broker.mse.streaming.group.by.flush.threshold (default 0, disabled).
• When set to a positive value, the broker injects it as the streamingGroupByFlushThreshold query option for MSE queries that don't already specify it.
• Per-query SET streamingGroupByFlushThreshold = N always wins via putIfAbsent, including SET streamingGroupByFlushThreshold = 0 to disable streaming group-by for a specific query when the cluster default is non-zero.
• Injection happens once after compilation but before the EXPLAIN/execute branch, so EXPLAIN PLAN FOR … reflects the same options the executed query would see.

Rollout / backwards compatibility

• Default is 0 → no behavior change for existing deployments. Existing query option still works exactly as before.
• Recommended starting value mirrors the PR Implement streaming group by leaf stage operator for multi-stage engine #18035 guidance (a few thousand groups); operators should tune based on observed leaf-stage cardinality.
• Mixed-broker rollouts are safe: brokers without the config simply don't inject the option, and servers handle the option the same way either way.
• The config takes effect on broker restart (resolved once in the constructor, like the other MSE broker defaults).

Test plan

• New unit tests in MultiStageBrokerRequestHandlerTest covering: injection when option absent, per-query SET=0 / SET=N overriding the broker default, no injection when config unset.
• ./mvnw spotless:apply checkstyle:check license:check -pl pinot-spi,pinot-broker clean.
• ./mvnw -pl pinot-broker -Dtest=MultiStageBrokerRequestHandlerTest test — 4/4 passing.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.69%. Comparing base (51bcec3) to head (adcc9ea).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...requesthandler/MultiStageBrokerRequestHandler.java	85.71%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18510      +/-   ##
============================================
+ Coverage     63.66%   63.69%   +0.02%     
  Complexity     1684     1684              
============================================
  Files          3266     3266              
  Lines        199903   199934      +31     
  Branches      31061    31069       +8     
============================================
+ Hits         127272   127349      +77     
+ Misses        62466    62423      -43     
+ Partials      10165    10162       -3     

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	63.69% <85.71%> (+0.02%)	⬆️
temurin	63.69% <85.71%> (+0.02%)	⬆️
unittests	63.69% <85.71%> (+0.02%)	⬆️
unittests1	55.79% <ø> (+<0.01%)	⬆️
unittests2	34.97% <85.71%> (+0.02%)	⬆️

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