fix(platform): fix image delete button on EditAgentForm (#12362)

### Summary
- SECRT-2094: Fix store image delete button accidentally submitting the
edit form — the remove image <button> in ThumbnailImages.tsx was missing
type="button", causing it to act as a form submit inside the
EditAgentForm. This closed the modal and showed a success toast without
the user clicking "Update submission".

https://github.com/user-attachments/assets/86cbdd7d-90b1-473c-9709-e75e956dea6b

###  Changes
- `frontend/.../ThumbnailImages.tsx` — added type="button" to image
remove button

autogpt_platform/frontend/src/components/contextual/PublishAgentModal/components/AgentInfoStep/components/ThumbnailImages.tsx

@@ -108,6 +108,7 @@ export function ThumbnailImages({
                 className="relative flex-shrink-0 overflow-visible"
               >
                 <button
+                  type="button"
                   onClick={() => handleRemoveImage(index)}
                   className="absolute -right-2 -top-2 z-50 inline-flex size-6 items-center justify-center rounded-full bg-slate-900"
                   aria-label="Remove image"