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));
     }
 
     /**

tests/mocha/json_test.js

@@ -275,7 +275,7 @@ suite('JSON Block Definitions', function() {
 
       const image1 = options[1][0];
       assertImageEquals(IMAGE1, image1);
-      chai.assert.equal(IMAGE1.alt, IMAGE1_ALT_TEXT);  // Via Msg reference
+      chai.assert.equal(image1.alt, IMAGE1_ALT_TEXT);  // Via Msg reference
       chai.assert.equal(VALUE1, options[1][1]);
 
       const image2 = options[2][0];