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.

[cello-guglielmi]

Hi sorry, the new terminal every time I run a python file has been a bit maddening and I came across this.

Just hoping we can get this implemented as it would improve my workflow a ton!

[eleanorjboyd]

Thank you for working on this—the goal of reusing an existing terminal is still valuable. Since this PR was opened, however, the terminal execution path has changed substantially, particularly with the queue-based execution work in #25669. As a result, the current implementation would need a significant rewrite to fit the architecture we use today.

I’m going to close this PR for now to keep the open queue current, but this is not a rejection of the feature. If you’re still interested in pursuing it, you’re very welcome to revise this PR or open a new one based on the latest main. Thanks again for the contribution!