Fix: Prevent use-after-free in HTTPRequest completion callback

[seer-by-sentry]

Fixes CLIENT-41R. The issue was that: HTTPRequest's std::function destructor accesses invalid captured lambda state, causing an access violation.

• The m_completionCallback is now moved to a local variable before being invoked.
• The m_completionCallback member is explicitly set to nullptr before the callback is executed.
• This prevents a potential use-after-free issue if the HTTPRequest object is deleted (e.g., by HTTPManager::Tick) immediately after the callback returns, which could destroy the callback's captured state while it's still on the call stack.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 11809149

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