Closes #42268
We've migrated user selections when a given workspace has a single
worktree (as then we could determine what the target worktree is).

Release Notes:

- python: Fixed selected virtual environments not being
persisted/deserialized correctly within long-running Zed sessions (where
multiple different projects might've been opened). This is a breaking
change for users of multi-worktree projects - your selected toolchain
for those projects will be reset.

Co-authored-by: Dino <dino@zed.dev>