Fix saveField to properly reject on failure

Previously when the content API returned a non-ok response,
saveField would set error state but still resolve the promise.
This caused downstream code to think save succeeded when it failed.

Now properly extracts error message from response and throws.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/visual-editing/toolbar.ts

@@ -721,6 +721,11 @@ export function renderToolbar(config: ToolbarConfig): string {
       } else {
         setSaveState("error");
         console.error("Save failed:", res.status);
+        // Reject the promise so callers know save failed
+        return res.json().then(function(e) {
+          var msg = (e && e.error && e.error.message) ? e.error.message : ("Save failed: " + res.status);
+          throw new Error(msg);
+        });
       }
     })
     .catch(function(err) {