MultiList: Add thread safety to GenericMultiListClass

[seer-by-sentry]

Fixes CLIENT-MN. The issue was that: Concurrent list destruction causes thread A to read from a MultiListNodeClass object freed by thread B.

• Added a FastCriticalSectionClass mutex to GenericMultiListClass to protect list operations.
• Modified all methods that modify the list structure (Add, Remove, Contains, etc.) to acquire the mutex before operating on the list and release it afterwards.
• Modified the MultiListObjectClass destructor to properly handle list removal in a thread-safe manner.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 2153410

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