Propagate stats retrieval failures in unused-index cleanup service

[Copilot]

This PR addresses the review feedback on discussion_r3303031990: UnusedIndexCleanupService::getAllManagedIndices() was swallowing getStats() exceptions and returning an empty result, which could falsely report “no unused indices.”
The change makes stats failures explicit by letting exceptions propagate to command-level error handling.

• Service behavior change

  • Removed local exception swallowing in UnusedIndexCleanupService::getAllManagedIndices().
  • SearchIndexServiceInterface::getStats() failures now bubble up instead of being converted to [].

• Focused unit coverage

  • Added a unit test asserting findUnusedIndices() propagates a thrown stats exception.

// before
try {
    $stats = $this->searchIndexService->getStats($indexPrefix . '*');
} catch (Exception) {
    return [];
}

// after
$stats = $this->searchIndexService->getStats($indexPrefix . '*');

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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