Animate Zeta button while generating completions (#22899)

Release Notes:

- N/A

Co-authored-by: Thorsten <thorsten@zed.dev>

crates/editor/src/inline_completion_tests.rs

@@ -387,6 +387,10 @@ impl InlineCompletionProvider for FakeInlineCompletionProvider {
         true
     }
 
+    fn is_refreshing(&self) -> bool {
+        false
+    }
+
     fn refresh(
         &mut self,
         _buffer: gpui::Model<language::Buffer>,