Fix flaky test failures in ShardsLimitAllocationDeciderIT

[cwperks]

Description

This PR is a similar attempt to #19762 to fix flaky tests in ShardsLimitAllocationDeciderIT

See example failure here: https://build.ci.opensearch.org/job/gradle-check/69672/testReport/junit/org.opensearch.cluster.routing.allocation.decider/ShardsLimitAllocationDeciderIT/testCombinedClusterAndIndexSpecificShardLimits__p0___opensearch_experimental_feature_writable_warm_index_enabled___true___/

>  ./gradlew ':server:internalClusterTest' --tests 'org.opensearch.cluster.routing.allocation.decider.ShardsLimitAllocationDeciderIT.testCombinedClusterAndIndexSpecificShardLimits' -Dtests.iters=5

java.lang.AssertionError: Total assigned shards should be 17 expected:<17> but was:<16>
	at __randomizedtesting.SeedInfo.seed([228B86D5A534EC39:A0832C873E69A8FB]:0)
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.failNotEquals(Assert.java:835)
	at org.junit.Assert.assertEquals(Assert.java:647)
	at org.opensearch.cluster.routing.allocation.decider.ShardsLimitAllocationDeciderIT.lambda$testCombinedClusterAndIndexSpecificShardLimits$0(ShardsLimitAllocationDeciderIT.java:303)
	at org.opensearch.test.OpenSearchTestCase.assertBusy(OpenSearchTestCase.java:1193)
	at org.opensearch.test.OpenSearchTestCase.assertBusy(OpenSearchTestCase.java:1166)
	at org.opensearch.cluster.routing.allocation.decider.ShardsLimitAllocationDeciderIT.testCombinedClusterAndIndexSpecificShardLimits(ShardsLimitAllocationDeciderIT.java:263)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
	at java.base/java.lang.reflect.Method.invoke(Method.java:565)
	at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1750)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$8.evaluate(RandomizedRunner.java:938)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$9.evaluate(RandomizedRunner.java:974)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:988)
	at org.opensearch.test.OpenSearchTestClusterRule$1.evaluate(OpenSearchTestClusterRule.java:369)
	at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
	at org.junit.rules.RunRules.evaluate(RunRules.java:20)

Related Issues

Resolves #19726

Stable with 100x runs. Fails a few runs consistently in a batch of 100 iters without this fix.

Check List

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

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Summary by CodeRabbit

• Bug Fixes
  • Improved reliability of cluster shard-allocation tests by waiting for cluster stabilization, forcing a reroute after index creation, tightening shard-distribution checks, and adding a 60s timeout to assertions to reduce flaky failures and improve test stability.

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📝 Walkthrough

Walkthrough

The PR adds a changelog entry and updates a test: ShardsLimitAllocationDeciderIT.testCombinedClusterAndIndexSpecificShardLimits now waits for cluster stabilization, forces a reroute after index creation, adjusts shard-distribution assertions to count only nodes that actually own shards, and wraps final assertions with a 60s timeout.

Changes

Cohort / File(s)	Summary
Test Stabilization & Assertions server/src/internalClusterTest/java/org/opensearch/cluster/routing/allocation/decider/ShardsLimitAllocationDeciderIT.java	Wait for cluster stabilization and call a forced reroute after creating indices; change shard-distribution check to count only nodes that own shards (expecting three nodes with shard counts 6, 6, and 5); wrap final assertions in a 60s timeout.
Changelog CHANGELOG.md	Added a Fixed entry noting flaky test failures in ShardsLimitAllocationDeciderIT with associated PR link.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• andrross
• reta
• dbwiddis
• kotwanikunal
• msfroh

Poem

🐰 I waited, I rerouted, I counted each shard,
Three nodes now balanced, their burdens not hard.
A timeout to guard the test's patient art,
I hop with a grin — the cluster's in part. 🥕✨

Pre-merge checks

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main fix being applied: resolving flaky test failures in ShardsLimitAllocationDeciderIT.
Description check	✅ Passed	The PR description comprehensively covers the problem, provides a failure example, references the related issue, and demonstrates the fix stability. All required template sections are present.
Linked Issues check	✅ Passed	The PR directly addresses the flaky test failures documented in issue #19726 by implementing fixes to the testCombinedClusterAndIndexSpecificShardLimits test method.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the flaky test: modifications to ShardsLimitAllocationDeciderIT test logic and a CHANGELOG entry documenting the fix.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[github-actions]

❌ Gradle check result for e8fba2b: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for e8fba2b: null

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❕ Gradle check result for bc221a8: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.26%. Comparing base (93ae8db) to head (ce82e1c).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #20375      +/-   ##
============================================
- Coverage     73.33%   73.26%   -0.07%     
+ Complexity    72125    72086      -39     
============================================
  Files          5798     5798              
  Lines        329654   329697      +43     
  Branches      47491    47508      +17     
============================================
- Hits         241741   241566     -175     
- Misses        68504    68757     +253     
+ Partials      19409    19374      -35     

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[github-actions]

✅ Gradle check result for ce82e1c: SUCCESS