Modify Run Selection/Line to reuse existing terminal

[s-kai273]

Summary

To fix #24960, I implemented a new function to reuse an existing terminal on Run Selection/Line.

What was done

• Implement the feature
• Implement test cases

[s-kai273]

@anthonykim1
Hi, I’ve implemented a rough version of this feature and confirmed that it works as expected in my local environment.

As for the basic implementation approach, I’ve created ExternalTerminalService that implements ITerminalService.
Since the existing TerminalService is designed to execute commands only within terminals it creates, my plan is to switch between the two based on the option value.

I’m aware that further refactoring is needed, but I’d like to confirm whether this overall implementation direction looks okay to you.