Fix a buggy C++ object pool implementation. The current code exhibits races, double-free risks, and resource leaks under concurrent borrow/return. Refactor it into a correct, thread-safe object pool template. Requirements: - Provide borrow() that blocks (with an optional timeout) when the pool is empty, creating up to a max capacity; ensure fairness and avoid spurious wakeups. - Provide returnObject(obj) that safely returns an object; reject invalid or duplicate returns; optionally reset objects before reuse. - Enforce RAII with a scoped handle that automatically returns the object on destruction; handle must be movable but not copyable. - Ensure safe shutdown: destructor stops producers/consumers, wakes waiters, and releases resources without deadlocks. - Avoid busy-waiting; use std::mutex, std::condition_variable, and std::atomic correctly; prevent ABA issues (e.g., with generation counters). - Include minimal unit tests demonstrating correctness under multiple threads. Assume C++17 or later and document the complexity of core operations.