Add test for stale writer correctness with pinned timestamp snapshots

[ashking94]

Description

This PR adds a unit test for handling the case of choosing the correct metadata file when there are metadata files created by stale writer which has higher timestamp than correct writer (which has higher primary term) but below the pinned timestamp. This test checks that the timestamp picked is always the one with highest primary term.

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]

❌ Gradle check result for 10a24b1: 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 10a24b1: 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?

[ashking94]

❌ Gradle check result for 10a24b1: 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?

Unrelated failures:
Test Result (2 failures / -4)
org.opensearch.remotestore.RemoteStorePinnedTimestampsGarbageCollectionIT.testLiveIndexNoPinnedTimestamps
org.opensearch.snapshots.SharedClusterSnapshotRestoreIT.testSnapshotFileFailureDuringSnapshot