Improve OnlineServices shutdown safety

[seer-by-sentry]

Fixes CLIENT-2SF. The issue was that: Worker thread's std::function callback implicitly captured a dangling this pointer from a deallocated NGMP_OnlineServicesManager instance, causing an access violation on invocation.

• Introduced an atomic flag m_bShuttingDown to NGMP_OnlineServicesManager to indicate shutdown status.
• Modified asynchronous callbacks in GetOnlineServiceConfig, CheckVersion, and DownloadPatcherFile to safely access NGMP_OnlineServicesManager member variables, preventing potential crashes during shutdown.
• Prevented the creation of new screenshot processing threads if NGMP_OnlineServicesManager is shutting down.
• Ensured all active screenshot threads are joined and completed during NGMP_OnlineServicesManager::Shutdown before proceeding with other cleanup.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 9520320

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