Analytics-engine: Update coordinator reduce to use search threadpool

[mch2]

Description

Run the coordinator reduce on the SEARCH pool instead of a virtual thread

This also properly wires up AbstractRunnable to handle rejection.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

PR Reviewer Guide 🔍

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🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

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⚡ No major issues detected

[bharath-techie]

LGTM

[Bukhtawar]

Looks good, we need to ensure DF doesn't become a bottleneck

[Bukhtawar]

uite: Test class org.opensearch.analytics.exec.stage.ReduceStageExecutionTests
  2> REPRODUCE WITH: ./gradlew ':sandbox:plugins:analytics-engine:test' --tests 'org.opensearch.analytics.exec.stage.ReduceStageExecutionTests.testBufferedStartInvokesReduceAndTerminalInvokesClose' -Dtests.seed=94836B2179BDF53D -Dtests.security.manager=true -Dtests.jvm.argline="-XX:TieredStopAtLevel=1 -XX:ReservedCodeCacheSize=64m" -Dtests.locale=en-FI -Dtests.timezone=Africa/Accra -Druntime.java=25
  2> java.lang.NullPointerException: Cannot invoke "java.util.concurrent.Executor.execute(java.lang.Runnable)" because "this.executor" is null
        at __randomizedtesting.SeedInfo.seed([94836B2179BDF53D:1D23DC907AA0ADD9]:0)
        at org.opensearch.analytics.exec.stage.coordinator.LocalTaskRunner.run(LocalTaskRunner.java:46)
        at org.opensearch.analytics.exec.stage.coordinator.LocalTaskRunner.run(LocalTaskRunner.java:25)
        at org.opensearch.analytics.exec.stage.ReduceStageExecutionTests.scheduleAndDispatch(ReduceStageExecutionTests.java:272)
        at org.opensearch.analytics.exec.stage.ReduceStageExecutionTests.testBufferedStartInvokesReduceAndTerminalInvokesClose(ReduceStageExecutionTests.java:76)

[github-actions]

✅ Gradle check result for f584b0d: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.38%. Comparing base (debf6ed) to head (f584b0d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #21756      +/-   ##
============================================
+ Coverage     73.33%   73.38%   +0.05%     
- Complexity    74996    75003       +7     
============================================
  Files          6012     6012              
  Lines        340934   340934              
  Branches      49076    49076              
============================================
+ Hits         250021   250198     +177     
+ Misses        71005    70752     -253     
- Partials      19908    19984      +76     

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