Fix unnecessary-mut-passed lint (#36490)

Release Notes:

- N/A

crates/assistant_context/src/context_store.rs

@@ -320,7 +320,7 @@ impl ContextStore {
                 .client
                 .subscribe_to_entity(remote_id)
                 .log_err()
-                .map(|subscription| subscription.set_entity(&cx.entity(), &mut cx.to_async()));
+                .map(|subscription| subscription.set_entity(&cx.entity(), &cx.to_async()));
             self.advertise_contexts(cx);
         } else {
             self.client_subscription = None;