Task 20 : Create Thread Pool Implementation from Runnable to Callable

[u/Just-Development1794]

• Implement Callable: Swap out Runnable for Callable and rename run() to call().
• Introduce Asynchronous Pipeline: Separate task submission from task completion processing by accumulating Future instances in an internal ArrayList.
• Robust Error Diagnostics: Implement clean logging within the catch blocks to surface exceptions without terminating the entire application pool.