fix: return self::FAILURE from CleanupUnusedIndicesCommand on exception

[Copilot]

When an exception was caught in CleanupUnusedIndicesCommand::execute(), the command printed the error but still exited 0, making failures invisible to automation (cron, CI, K8s jobs).

Change

• catch block now returns self::FAILURE after writing the error message
• finally still runs unconditionally, so the lock is always released

} catch (Exception $e) {
    $output->writeln('<error>' . $e->getMessage() . '</error>');
    return self::FAILURE; // was missing; caused silent failure
} finally {
    $this->release();
}

[sonarqubecloud]

Quality Gate failed

Failed conditions
2 New Major Issues (required ≤ 0)

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