Thread-Safe Singleton

In the Singleton problem, you implemented the pattern using __new__. That implementation has a race condition: two threads can simultaneously find _instance is None, both proceed past the check, and both create an instance — breaking the Singleton guarantee.

Implement a thread-safe ThreadSafeSingleton class that:

• Uses a class-level threading.Lock to protect instance creation
• Returns the same instance on every call to ThreadSafeSingleton(), even under concurrent access
• Provides reset() as a classmethod to clear the stored instance (for testing only)

Constraints:

• You must override __new__
• The lock must be a class attribute (not created inside __new__)
• Do not use threading.RLock, threading.Semaphore, or any other primitive — use threading.Lock
• reset() does not need to acquire the lock (it is only called from single-threaded test code)