Add label_for_completion to extension API (#10175)

This PR adds the ability for extensions to implement
`label_for_completion` to customize completions coming back from the
language server.

We've used the Gleam extension as a motivating example, adding
`label_for_completion` support to it.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/language/src/language.rs

@@ -213,8 +213,9 @@ impl CachedLspAdapter {
         &self,
         completion_items: &[lsp::CompletionItem],
         language: &Arc<Language>,
-    ) -> Vec<Option<CodeLabel>> {
+    ) -> Result<Vec<Option<CodeLabel>>> {
         self.adapter
+            .clone()
             .labels_for_completions(completion_items, language)
             .await
     }
@@ -385,10 +386,10 @@ pub trait LspAdapter: 'static + Send + Sync {
     async fn process_completions(&self, _: &mut [lsp::CompletionItem]) {}
 
     async fn labels_for_completions(
-        &self,
+        self: Arc<Self>,
         completions: &[lsp::CompletionItem],
         language: &Arc<Language>,
-    ) -> Vec<Option<CodeLabel>> {
+    ) -> Result<Vec<Option<CodeLabel>>> {
         let mut labels = Vec::new();
         for (ix, completion) in completions.into_iter().enumerate() {
             let label = self.label_for_completion(completion, language).await;
@@ -397,7 +398,7 @@ pub trait LspAdapter: 'static + Send + Sync {
                 *labels.last_mut().unwrap() = Some(label);
             }
         }
-        labels
+        Ok(labels)
     }
 
     async fn label_for_completion(

crates/language/src/language_registry.rs

@@ -746,7 +746,10 @@ impl LanguageRegistry {
                     let capabilities = adapter
                         .as_fake()
                         .map(|fake_adapter| fake_adapter.capabilities.clone())
-                        .unwrap_or_default();
+                        .unwrap_or_else(|| lsp::ServerCapabilities {
+                            completion_provider: Some(Default::default()),
+                            ..Default::default()
+                        });
 
                     let (server, mut fake_server) = lsp::FakeLanguageServer::new(
                         server_id,