Adds discussion to the optimistic design response

What does this MR do?

This adds a discussion object to the optimisticResponse when we upload an image to the designs section. Doing so prevents a warning from appearing in the console.

Addresses #33306 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers