toolchains: Run listing tasks on background thread (#21414)

Potentially fixes #21404

This is a speculative fix, as while I was trying to repro this issue
I've noticed that introducing artificial delays in ToolchainLister::list
could impact apps responsiveness. These delays were essentially there to
stimulate PET taking a while to find venvs.

Release Notes:

- Improved app responsiveness in environments with multiple Python
virtual environments

crates/language/src/toolchain.rs

@@ -24,7 +24,7 @@ pub struct Toolchain {
     pub as_json: serde_json::Value,
 }
 
-#[async_trait(?Send)]
+#[async_trait]
 pub trait ToolchainLister: Send + Sync {
     async fn list(
         &self,