🔒 fix: prevent out-of-bounds writes in unmanaged collections

[myarichuk]

🎯 What: The vulnerability fixed
A potential buffer overflow (out-of-bounds write) in ArenaList.Add and ArenaPtrStack.Push. The collections previously indexed the backing buffer using _header->Count++ immediately after checking if growth was needed. If a race condition occurred or Grow() failed to update the capacity properly, this could lead to memory corruption.

⚠️ Risk: The potential impact if left unfixed
An attacker (or even an unintended race condition) could cause the collection to write outside its allocated bounds, leading to unmanaged memory corruption, crashes, or potentially arbitrary code execution if the corrupted memory is later interpreted as executable instructions or object references.

🛡️ Solution: How the fix addresses the vulnerability
The index is now cached locally before use (int index = _header->Count;), rigorously validated against the current capacity (if ((uint)index >= (uint)_header->Capacity)), and only updated and written to if the bounds check passes. This ensures memory safety even if the internal state is concurrently modified or logically inconsistent, throwing an InvalidOperationException instead of corrupting memory.

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PR created automatically by Jules for task 2143493459154911632 started by @myarichuk

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