Add CompletionStage variants to ClusterAdminClient as an alternative to ActionListener

[cwperks]

Description

Similar to #19161 and #18998 this PR applies the same concept to ClusterAdminClient.

One of my eventual goals is to deprecate and remove anything from these interfaces that returns an ActionFuture to discourage any usage of blocking actions. Even though these signatures are primarily used in tests, I don't think they should be part of the interfaces.

Check List

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

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

📜 Recent review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 07e455c and 22550b1.

📒 Files selected for processing (1)

• CHANGELOG.md

🚧 Files skipped from review as they are similar to previous changes (1)

• CHANGELOG.md

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📝 Walkthrough

Walkthrough

Adds CompletionStage-returning async helper methods to ClusterAdminClient that wrap existing ActionListener-based calls using CompletableFuture, updates a test to use the new stateAsync API, and adds an Unreleased changelog entry documenting the additions.

Changes

Cohort / File(s)	Summary
Changelog CHANGELOG.md	Added Unreleased 3.x entry documenting CompletionStage variants for ClusterAdminClient (PR reference).
Cluster admin async API server/src/main/java/org/opensearch/transport/client/ClusterAdminClient.java	Added many public default methods (CompletionStage-returning) for cluster operations (health, state, settings, reroute, nodes info/stats/usage/hotthreads, cluster stats, tasks, search shards, repositories, snapshots, pipelines, stored scripts, dangling indices, weighted routing, decommissioning, search pipelines). Each method creates a CompletableFuture and delegates to existing APIs via ActionListener.wrap. New imports for CompletableFuture and CompletionStage. Review attention: concurrency/exception propagation and API contract consistency.
Test update server/src/test/java/org/opensearch/ratelimitting/admissioncontrol/AdmissionControlSingleNodeTests.java	Replaced use of ActionFuture for cluster state with stateAsync(...).toCompletableFuture().join() and added null-checks; removed unused imports. Verify test timing/exception behavior changes.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: adding CompletionStage variants to ClusterAdminClient as an alternative to ActionListener.
Description check	✅ Passed	The description includes context about related PRs, explains the intent to modernize the API, and covers most checklist items, though testing and documentation boxes remain unchecked.
Docstring Coverage	✅ Passed	Docstring coverage is 92.00% which is sufficient. The required threshold is 80.00%.

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[github-actions]

❌ Gradle check result for 07e455c: 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 07e455c: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 2.22222% with 132 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.27%. Comparing base (93ae8db) to head (22550b1).
⚠️ Report is 145 commits behind head on main.

Files with missing lines	Patch %	Lines
...pensearch/transport/client/ClusterAdminClient.java	2.22%	132 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #20509      +/-   ##
============================================
- Coverage     73.33%   73.27%   -0.06%     
- Complexity    72125    72144      +19     
============================================
  Files          5798     5799       +1     
  Lines        329654   329858     +204     
  Branches      47491    47518      +27     
============================================
- Hits         241741   241715      -26     
- Misses        68504    68778     +274     
+ Partials      19409    19365      -44     

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[github-actions]

✅ Gradle check result for 22550b1: SUCCESS

[peterzhuamazon]

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This PR is stalled because it has been open for 30 days with no activity.