fix(cookbook): open() no longer crashes when a task has a diagnosis (#4417)

_showDiagnosis referenced an undefined `body` (left over from the refactor
that moved the diagnosis text into the toolbar), throwing a ReferenceError
whenever a failed task rendered fix buttons. Because open() wraps its render
in try/finally with no catch, the throw escaped before the modal was
un-hidden, so the whole Cookbook silently failed to open.

- cookbook-diagnosis.js: append the fixes row to `diag` (the in-scope
  container) instead of the removed `body` element.
- cookbook.js: guard the render passes in open() so one broken task card
  can't leave the entire panel stuck hidden.

Fixes #4406

static/js/cookbook-diagnosis.js

@@ -757,7 +757,7 @@ export function _showDiagnosis(panel, diagnosis, sourceText) {
       });
       row.appendChild(btn);
     }
-    body.appendChild(row);
+    diag.appendChild(row);
   }
 }