Add Remote Store integration test infrastructure with SearchOnlyActionIT

[lawofcycles]

Description

Add integration test infrastructure for running ISM tests against a Remote Store enabled cluster with warm and search node roles.

This PR introduces a remoteStoreIntegTest Gradle task with a dedicated test cluster configured with:

• Remote Store enabled (fs type repository for segment, translog, and state)
• Segment Replication enabled
• Node 0: [cluster_manager, data, warm]
• Node 1: [search] (requires Add cluster.initial_cluster_manager_nodes to testClusters OVERRIDABLE_SETTINGS OpenSearch#20348 for cluster.initial_cluster_manager_nodes override)

As the first test on this infrastructure, SearchOnlyActionIT validates the search_only ISM action, which requires Remote Store + Segment Replication + search replicas to pass validation (ValidateSearchOnly). This serves as both a functional test and a verification that the test infrastructure works correctly.

Additionally, IndexManagementRestTestCase is updated to ignore Segment Replication background tasks (segrep_publish_checkpoint, indices:admin/publishCheckpoint) during test teardown, as these tasks run continuously on Remote Store clusters.

This task is not yet included in CI workflows. As more Remote Store specific tests are added, it may be worth considering adding it to CI.
For now it can be run ad hoc with

./gradlew remoteStoreIntegTest

Related Issues

Resolves #1564

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.44%. Comparing base (9491be7) to head (be6057b).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1589      +/-   ##
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+ Coverage   76.32%   76.44%   +0.11%     
==========================================
  Files         379      381       +2     
  Lines       17942    18069     +127     
  Branches     2479     2503      +24     
==========================================
+ Hits        13695    13813     +118     
- Misses       2982     2991       +9     
  Partials     1265     1265              

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