HTTPRequest: Prevent use-after-free in HTTPRequest callbacks during shutdown

[seer-by-sentry]

Fixes CLIENT-NZ. The issue was that: HTTPRequest is deleted immediately after invoking its completion callback, causing a use-after-free crash in the std::function destructor.

• Added a boolean parameter bInvokeCallback to Threaded_SetComplete to control callback invocation.
• Modified HTTPRequest::Threaded_SetComplete to clear the completion and progress callbacks when bInvokeCallback is false to prevent use-after-free issues during shutdown.
• Modified HTTPManager::MultiPerform to call Threaded_SetComplete with bInvokeCallback = false during shutdown.
• Modified HTTPRequest::InvokeDelayAction and HTTPManager::MultiPerform to call Threaded_SetComplete with bInvokeCallback = true during normal operation (Tick).

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 2250123

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