fix: Deep copy options used by dropdown fields (#6425)

This uncovered a broken test that was only passing because of a side effect of the dropdown option data getting modified.

core/field_dropdown.ts

@@ -125,6 +125,8 @@ export class FieldDropdown extends Field {
 
     if (Array.isArray(menuGenerator)) {
       validateOptions(menuGenerator);
+      // Deep copy the option structure so it doesn't change.
+      menuGenerator = JSON.parse(JSON.stringify(menuGenerator));
     }
 
     /**