python: Look for local venvs in all directories between root of the worktree and current pyproject.toml (#37037)

cc @michael-ud - if you can build Zed, I'd appreciate it if you could
give this a go with your project. Otherwise I can provide a link to
download of current nightly via an e-mail for you to try out (if you
want).
This change will land in Preview (if merged) on next Wednesday and then
it'll be in Stable a week after that.
Related to: #20402
Release Notes:

- python: Zed now searches for virtual environments in intermediate
directories between a root of the worktree and the location of
pyproject.toml applicable to the currently focused file.

crates/language/src/toolchain.rs

@@ -52,7 +52,7 @@ pub trait ToolchainLister: Send + Sync {
     async fn list(
         &self,
         worktree_root: PathBuf,
-        subroot_relative_path: Option<Arc<Path>>,
+        subroot_relative_path: Arc<Path>,
         project_env: Option<HashMap<String, String>>,
     ) -> ToolchainList;
     // Returns a term which we should use in UI to refer to a toolchain.