Rename LSP function and simplify tests (#27313)

While working on a fix I found opportunities to improve readability, but
it's a big rename diff, so I'm landing separately.

Release Notes:

- N/A

crates/extension_host/src/extension_store_test.rs

@@ -670,7 +670,7 @@ async fn test_extension_store_with_test_extension(cx: &mut TestAppContext) {
     );
 
     // The extension creates custom labels for completion items.
-    fake_server.handle_request::<lsp::request::Completion, _, _>(|_, _| async move {
+    fake_server.set_request_handler::<lsp::request::Completion, _, _>(|_, _| async move {
         Ok(Some(lsp::CompletionResponse::Array(vec![
             lsp::CompletionItem {
                 label: "foo".into(),