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

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 2baa1ab: 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?