Language Runtime terminals incorrectly restored after quitting

[jmcphers]

Quitting Positron should also end any Language Runtime sessions. While the sessions do exit when Positron does, they are brought back in a bad state when it is next opened.

This happens since we're piggybacking on the existing Terminal infrastructure; VS Code attempts to reload the "persistent" terminals by replaying the shell command used to start them, creating an orphaned Runtime.

We want the Language Runtime terminals to have the same lifetime as the PtyHost. This does not seem to be possible with the existing Terminal API; its isTransient flag controls both restart and reload behavior. Language Runtime terminals should persist through reload, but not restart.

[petetronic]

A possible solution is as follows. When the window is being reloaded, the terminal service only detaches from the terminal processes and skips the serialize/persistence flow that it does on shutdown.

During shutdown, the ptyService is the one responsible for serializing the terminal state (but remembering it does not do this on reload). It is there we could detect which terminal processes are Positron language runtime kernel processes, and skip serialization for just those ones.

To avoid changing the existing terminal API, we could perhaps look for the existence of an ENV variable that is positron specific, and use that to mark which processes to skip serializing.