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/languages/src/python.rs

@@ -536,7 +536,7 @@ fn env_priority(kind: Option<PythonEnvironmentKind>) -> usize {
     }
 }
 
-#[async_trait(?Send)]
+#[async_trait]
 impl ToolchainLister for PythonToolchainProvider {
     async fn list(
         &self,