Terminals created for session stick around unnecessarily

Testing https://github.com/microsoft/vscode/issues/327169

I believe the above issue is fixed but I still believe some terminals are being eagerly created and sticking around unnecessarily.

Repro Steps (In Agents Window):
1. Create new session
2. Select an older session in the sessions list
3. Create new session
4. Select a different older session in the sessions list
5. Repeat for 5 or so sessions
6. Run `Inspect Agent Host Subscriptions`

Notice there are 6+ terminals listed even though I never interacted with any sessions or their terminals.