Ensure inlay hint toggling with modifiers happens fast (#25852)

Follow-up of https://github.com/zed-industries/zed/pull/25766 
Fixes the bugs found:

* modifier toggle not happening instantly due to `edit_debounce_ms`
considered
* hint update race that ignored the cache clear

Release Notes:

- N/A

crates/editor/src/inlay_hint_cache.rs

@@ -581,6 +581,7 @@ impl InlayHintCache {
             self.version += 1;
         }
         self.update_tasks.clear();
+        self.refresh_task = Task::ready(());
         self.hints.clear();
     }