Fix thread safety issues in producer-consumer implementation

[bkk07]

This PR fixes thread-safety issues in the producer-consumer implementation.

The previous implementation accessed a shared LinkedList concurrently without proper synchronization. Queue condition checks and wait/notify coordination were performed outside a shared lock, which could lead to race conditions, missed notifications, visibility issues, and potential queue corruption when multiple producers and consumers executed concurrently.

Changes made:

• Moved synchronization responsibility into DataQueue
• Guarded queue operations using synchronized methods
• Replaced unsafe condition checks with guarded wait loops
• Removed separate wait/notify coordination from Producer and Consumer
• Added proper handling for spurious wakeups using while loops
• Added volatile visibility guarantees for running flags

This preserves the original educational intent of demonstrating wait/notify based producer-consumer coordination while ensuring thread-safe behavior.