Reuse existing language servers for invisible worktrees (#30707)

Closes https://github.com/zed-industries/zed/issues/20767

Before:


https://github.com/user-attachments/assets/6438eb26-796a-4586-9b20-f49d9a133624


After:



https://github.com/user-attachments/assets/b3fc2f8b-2873-443f-8d80-ab4a35cf0c09



Release Notes:

- Fixed external files spawning extra language servers

crates/gpui/src/platform/test/platform.rs

@@ -236,7 +236,7 @@ impl Platform for TestPlatform {
     fn quit(&self) {}
 
     fn restart(&self, _: Option<PathBuf>) {
-        unimplemented!()
+        //
     }
 
     fn activate(&self, _ignoring_other_apps: bool) {