Allow completing slash command arguments from extensions (#13240)

This PR extends the extension API with support for completing slash
command arguments for slash commands defined in extensions.

Release Notes:

- N/A

crates/extension/src/extension_slash_command.rs

@@ -36,13 +36,34 @@ impl SlashCommand for ExtensionSlashCommand {
     }
 
     fn complete_argument(
-        &self,
-        _query: String,
+        self: Arc<Self>,
+        query: String,
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
-        _cx: &mut AppContext,
+        cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>> {
-        Task::ready(Ok(Vec::new()))
+        cx.background_executor().spawn(async move {
+            self.extension
+                .call({
+                    let this = self.clone();
+                    move |extension, store| {
+                        async move {
+                            let completions = extension
+                                .call_complete_slash_command_argument(
+                                    store,
+                                    &this.command,
+                                    query.as_ref(),
+                                )
+                                .await?
+                                .map_err(|e| anyhow!("{}", e))?;
+
+                            anyhow::Ok(completions)
+                        }
+                        .boxed()
+                    }
+                })
+                .await
+        })
     }
 
     fn run(

crates/extension/src/wasm_host/wit.rs

@@ -257,6 +257,21 @@ impl Extension {
         }
     }
 
+    pub async fn call_complete_slash_command_argument(
+        &self,
+        store: &mut Store<WasmState>,
+        command: &SlashCommand,
+        query: &str,
+    ) -> Result<Result<Vec<String>, String>> {
+        match self {
+            Extension::V007(ext) => {
+                ext.call_complete_slash_command_argument(store, command, query)
+                    .await
+            }
+            Extension::V001(_) | Extension::V004(_) | Extension::V006(_) => Ok(Ok(Vec::new())),
+        }
+    }
+
     pub async fn call_run_slash_command(
         &self,
         store: &mut Store<WasmState>,