feat: add onBackgroundRefreshFailed hook to AbstractUseStaleRequest

[an-adamchuk]

Release Notes

Adds an extensibility hook to AbstractUseStaleRequest so consumers can observe background refresh failures without the library forcing specific behaviour.

Technical Notes

The background refresh closure previously swallowed all \Throwable silently:

} catch (\Throwable) {
    // Nobody cares, this is literally what use-stale is good at
}

The new no-op hook:

} catch (\Throwable $e) {
    $reRequest->onBackgroundRefreshFailed($e);
}

protected function onBackgroundRefreshFailed(\Throwable $e): void {}

Consumers override onBackgroundRefreshFailed() in their subclass or via a trait to add logging or metrics. The library itself stays behaviour-neutral.

Corresponding Tickets

MR-7177

Checklist

• PR is ready to review
• Enough tests are added to feel fully confident
• Product / Stakeholder signoff