Fix keyboard interrupt handling in download function

[phanimullapudi]

Description
This PR addresses issue #1382 by implementing proper keyboard interrupt handling for yfinance's download function, especially when using threaded downloads.

Problem
When running yf.download() with multiple tickers and threads enabled, keyboard interruptions (Ctrl+C) were not being properly caught, resulting in two issues:

• The download couldn't be cleanly interrupted
• The user had to press Ctrl+C multiple times or force-quit the application

Solution

• Added a global flag to track cancellation requests
• Implemented a signal handler for SIGINT (Ctrl+C)
• Modified download logic to check for cancellation and gracefully exit
• Ensured that partial results are returned to the user when interrupted
• Added comprehensive tests to verify interrupt handling behavior

[ValueRaider]

Looks fine just move to dev branch #1084