OnlineServices: Implement graceful shutdown of HTTP requests

[seer-by-sentry]

Fixes CLIENT-QE. The issue was that: HTTP request callback invoked during shutdown accesses destroyed manager object captured by value, causing an access violation.

• Modified HTTPRequest to allow skipping callback invocation during completion, controlled by a new m_bSkipCallback flag.
• Modified Threaded_SetComplete to accept a bSkipCallback parameter, setting the flag accordingly.
• Modified HTTPManager to skip callback invocation when completing requests during shutdown to prevent use-after-free issues.
• Added safety checks in the HTTPRequest destructor to ensure the HTTPManager is still valid before attempting to remove the handle from the multi-handle.
• Modified the artificial delay logic to invoke callbacks during normal operation.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 2352235

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