Fix NPE in validateSearchableSnapshotRestorable when shard size is unavailable

[lawofcycles]

Description

This PR fixes a NullPointerException that occurs when restoring remote snapshots when shard size information is not available in the ClusterInfo cache.

The validateSearchableSnapshotRestorable method in RestoreService attempts to calculate the total size of existing remote snapshot shards.
When ClusterInfo.getShardSize() returns null (which is valid behavior when shard size info is unavailable), the stream operation mapToLong(Long::longValue) throws a NullPointerException.

Related Issues

Resolves #19349

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 c2f7eb8: 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?

[lawofcycles]

The CI failure appears to be unrelated to my changes. The error is occurring in the Azure fixture Docker containers:

ERROR: for azure-fixture_azure-fixture-repositories-metering_1  cannot stop container: 
container PID is zombie and can not be killed

This seems to be an infrastructure/flaky test issue rather than a problem with the NPE fix in RestoreService.

[github-actions]

❌ Gradle check result for 2baa1ab: FAILURE

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[github-actions]

❌ Gradle check result for 2baa1ab: FAILURE

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[sandeshkr419]

Thanks for fixing this.

LGTM with a small rename suggestion. Can you please address this and rebase.

[github-actions]

❌ Gradle check result for e76f134: FAILURE

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[lawofcycles]

Apologies for the force push confusion. I accidentally included unrelated commits while cleaning up the merge commit, which notified many code owners unnecessarily.
PR is now properly rebased, so please check for merge.
Thanks

[github-actions]

✅ Gradle check result for 70f49a0: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 21.42857% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.21%. Comparing base (f3e6399) to head (b853af7).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../java/org/opensearch/snapshots/RestoreService.java	21.42%	8 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19684      +/-   ##
============================================
- Coverage     73.21%   73.21%   -0.01%     
+ Complexity    71564    71549      -15     
============================================
  Files          5785     5785              
  Lines        326759   326767       +8     
  Branches      47291    47294       +3     
============================================
- Hits         239246   239238       -8     
- Misses        68272    68284      +12     
- Partials      19241    19245       +4     

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[github-actions]

✅ Gradle check result for 597670a: SUCCESS

[github-actions]

✅ Gradle check result for 410142b: SUCCESS

[github-actions]

❌ Gradle check result for b853af7: FAILURE

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[github-actions]

✅ Gradle check result for b853af7: SUCCESS