Run in terminal: Handle dynamically switching between a background/watch terminal and a regular one

[Tyriar]

The agent can sometimes launch watch tasks and hang indefinitely, or at least until the command is interrupted:

Ideally I think we would handle this case and "downgrade" it to the background terminal concept in Copilot, then create a new terminal for that session as needed.

Related: https://github.com/microsoft/vscode-copilot/issues/16430

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This issue is being used to track generic problems with background terminals. We plan on having a holistic look at how background terminals work soon.

[Tyriar]

An issue I've seen reported a bunch are multiple terminals getting launched when they should be shared. I spoke with @pierceboggan and the reason he hit it was because the last one launched as a background terminal.

Repro for the error case:

1. Make sure node isn't installed
2. Install node via chat
3. Get it to add node to the path
4. Run npm run dev, 🐛 which goes to a (new) background terminal, this doesn't have the env so it errors out in addition to creating an unexpected terminal

Ideally this would transition the terminal to a background terminal at step 4, if the command happened to finish it would transition back to a non-background one.