Autocomplete commands that don't require access to workspace in prompt library (#12674)

This is useful to autocomplete prompts when writing a new one in the
prompt library.

Release Notes:

- N/A

crates/assistant/src/slash_command/file_command.rs

@@ -101,10 +101,10 @@ impl SlashCommand for FileSlashCommand {
         &self,
         query: String,
         cancellation_flag: Arc<AtomicBool>,
-        workspace: WeakView<Workspace>,
+        workspace: Option<WeakView<Workspace>>,
         cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>> {
-        let Some(workspace) = workspace.upgrade() else {
+        let Some(workspace) = workspace.and_then(|workspace| workspace.upgrade()) else {
             return Task::ready(Err(anyhow!("workspace was dropped")));
         };